Handover analysis for a moving vehicle

ABSTRACT

An apparatus and method are provided for performing a handover analysis. The apparatus comprises base station location identifying circuitry to obtain base station location information for a plurality of base stations that provide a wireless network for communication with a moving vehicle. In addition, moving vehicle tracking circuitry is provided to obtain position and velocity information for the moving vehicle. Handover metrics computation circuitry is then used to generate at least one handover metric computed from the position and velocity information for the moving vehicle and the base station location information, for use in determining a target base station in said plurality to be used when performing a handover procedure to transition communication with the moving vehicle from the current base station in said plurality to the target base station. By such an approach, this enables a variety of handover metrics to be generated that take into account the deployment of the wireless network, which can be useful in systems such as Air to Ground (ATG) systems where the moving vehicles have a relatively high velocity, and the base stations may be relatively far apart. Such an approach can enhance the algorithms used to evaluate the decision to trigger handover from one base station to another base station.

PRIORITY APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/814,563, filed on Mar. 10, 2020, which application claimspriority to GB Application No. 1910318.3, filed Jul. 17, 2019, and to GBApplication No. 1905222.4, filed Apr. 12, 2019, and to GB ApplicationNo. 1903217.6, filed Mar. 11, 2019. These applications are incorporatedby reference herein.

BACKGROUND

The present technique relates to the field of wireless communications.

It is known to provide air-to-ground (ATG) communication systems forcommunication between moving aircraft and a network of ground stations.Such systems can, for example, be used to provide a Wi-Fi hotspot withinthe aircraft in order to provide connectivity to passengers in theaircraft. With increasing demands for higher capacity, there is a desireto support modern telecommunications Standards such as 4G (LTE) in ATGsystems. However, this presents a number of technical issues.

In particular, the aircraft will typically be moving at high speed, andthe ground stations can be placed a relatively long distance apart, andthese factors can give rise to a number of issues when seeking tosupport modern telecommunications Standards such as 4G (LTE). One issuethat can arise is performance of a handover procedure to transition theaircraft's communication from one ground station to another groundstation. The standard metrics used in 4G wireless technologies forevaluating the decision to trigger the handover procedure are typicallylimited to radio signal strength metrics like RSRP (Reference SignalReceived Power) and RSRQ (Reference Signal Received Quality). However,in ATG systems it has been found that reliance on such metrics can leadto sub-optimal handover decisions being taken, and accordingly it wouldbe desirable to provide an improved mechanism for performing a handoveranalysis in such systems.

SUMMARY

In accordance with one example arrangement, there is provided anapparatus comprising: base station location identifying circuitry toobtain base station location information for a plurality of basestations that provide a wireless network for communication with a movingvehicle; moving vehicle tracking circuitry to obtain position andvelocity information for the moving vehicle; and handover metricscomputation circuitry to generate at least one handover metric computedfrom the position and velocity information for the moving vehicle andthe base station location information, for use in determining a targetbase station in said plurality to be used when performing a handoverprocedure to transition communication with the moving vehicle from acurrent base station in said plurality to the target base station.

In accordance with a further example arrangement, there is provided amethod of performing a handover analysis, comprising: obtaining basestation location information for a plurality of base stations thatprovide a wireless network for communication with a moving vehicle;obtaining position and velocity information for the moving vehicle; andemploying handover metrics computation circuitry to generate at leastone handover metric computed from the position and velocity informationfor the moving vehicle and the base station location information, foruse in determining a target base station in said plurality to be usedwhen performing a handover procedure to transition communication withthe moving vehicle from a current base station in said plurality to thetarget base station.

In accordance with a still further example arrangement, there isprovided an apparatus comprising: base station location identifyingmeans for obtaining base station location information for a plurality ofbase stations that provide a wireless network for communication with amoving vehicle; moving vehicle tracking means for obtaining position andvelocity information for the moving vehicle; and handover metricscomputation means for generating at least one handover metric computedfrom the position and velocity information for the moving vehicle andthe base station location information, for use in determining a targetbase station in said plurality to be used when performing a handoverprocedure to transition communication with the moving vehicle from acurrent base station in said plurality to the target base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technique will be described further, by way of illustrationonly, with reference to examples thereof as illustrated in theaccompanying drawings, in which:

FIG. 1 is a diagram schematically illustrating an air-to-ground (ATG)communication between an aircraft and a ground station;

FIG. 2 schematically illustrates the format of a communication frameused in one example implementation;

FIG. 3 is a block diagram illustrating components provided within avehicle terminal and a ground terminal in accordance with one examplearrangement;

FIG. 4 is a flow diagram illustrating a process performed by the vehicleterminal to determine the transmission frequency (f_(t)) of atransmitted signal;

FIG. 5 is a flow diagram showing an example of a Doppler adjustmentprocess performed by the vehicle terminal to determine the transmissionfrequency (f_(t)) of the transmitted signal;

FIG. 6 schematically shows an example of how the Doppler effect affectssignals from the ground station, and how the received signal can be usedto determine the transmission frequency (f_(t)) of the transmittedsignal;

FIG. 7 is a flow diagram showing another example of a Doppler adjustmentprocess performed by the vehicle terminal to determine the transmissionfrequency (f_(t)) of the transmitted signal;

FIGS. 8 and 9 schematically show examples of components in the vehicleterminal, used in the process of determining the transmission frequency(f_(t)) of the transmitted signal;

FIG. 10A illustrates how a connection setup signal (a RACH signal) canbe successfully communicated from a vehicle terminal to a groundterminal using the communication frame of FIG. 2 provided the vehicleterminal is no more than 108 km from the ground terminal;

FIG. 10B illustrates how when the distance between the vehicle terminaland the ground terminal exceeds 108 km the connection setup signal willnot be successfully received by the ground terminal when adopting thescheme of FIG. 10A;

FIG. 11 is a flow diagram illustrating a process performed by thevehicle terminal in accordance with one example implementation, in orderto ensure that the connection setup signal is successfully received bythe ground terminal within an identified timing window even when thedistance exceeds a setup threshold distance;

FIGS. 12A and 12B illustrate how the approach described in FIG. 11ensures correct reception of the connection setup signal, and enablesthe provision of a suitable response from the ground terminal thatallows a correct timing advance to be applied for future uplinkcommunication to the ground terminal;

FIG. 13 is a flow diagram illustrating how step 1055 of FIG. 11 may beperformed in accordance with one example implementation;

FIG. 14 is a flow diagram illustrating how step 1070 of FIG. 11 may beperformed in one example implementation;

FIG. 15 is a diagram schematically illustrating a scheduling issue thatcan arise when the vehicle terminal is separated from the groundterminal by a distance exceeding a scheduling threshold distance;

FIGS. 16A and 16B are a flow diagram illustrating a process performed bythe ground terminal in order to resolve the scheduling issue illustratedin FIG. 15, in accordance with one example arrangement;

FIGS. 17A to 17C illustrate how the process of FIGS. 16A and 16B may beapplied for various separation distances between the vehicle terminaland the ground terminal, in accordance with one example arrangement;

FIG. 18 illustrates multiple communication frame formats that can besupported in one example implementation; and

FIG. 19 is a flow diagram illustrating how the ground terminal in oneexample implementation can switch between the communication frameformats of FIG. 18 as separation distances permit, in order to seek toincrease the proportion of the communication frame available fordownlink communications;

FIG. 20 is a block diagram schematically illustrating the use of a highvelocity mobility manager within an ATG system in accordance with oneexample implementation;

FIG. 21 schematically illustrates an aircraft in flight over a grid ofground stations;

FIGS. 22 to 25 schematically illustrate four criteria that can be takeninto account, either alone or in combination, when making handoverdecisions in accordance with the technique described herein;

FIG. 26 is a flow diagram illustrating a handover analysis operationperformed in accordance with one example implementation;

FIGS. 27 to 30 are flow diagrams illustrating the production of handovermetrics for each of the four criteria illustrated schematically in FIGS.22 to 25, in accordance with one example implementation;

FIG. 31 identifies how a handover result for a particular base stationmay be computed by using the generated handover metrics, in accordancewith one example implementation; and

FIG. 32 is a flow diagram illustrating an operation performed byhandover decision circuitry in accordance with one exampleimplementation.

DESCRIPTION OF EXAMPLES

In accordance with the techniques described herein an apparatus isprovided that comprises base station location identifying circuitry toobtain base station location information for a plurality of basestations that provide a wireless network for communication with a movingvehicle, and moving vehicle tracking circuitry to obtain position andvelocity information for the moving vehicle. In a typicalimplementation, the base stations will be static, and accordingly theirlocation information will be fixed. In such implementations, the basestation location identifying circuitry may be arranged to have access toa database storing the location information for the various basestations. With regard to the moving vehicle, there are a number of waysin which the position and velocity information may be provided to themoving vehicle tracking circuitry. For example, this information may beobtained via reporting information provided to the moving vehicletracking circuitry by the moving vehicle, or alternatively the movingvehicle tracking circuitry may have access to an existing trackingfacility for the moving vehicle. For example, in implementations wherethe moving vehicle is an aircraft, then flight tracking systems alreadyexist that can be accessed to provide the position and velocityinformation for the aircraft.

The apparatus further comprises handover metrics computation circuitryto generate at least one handover metric computed from the position andvelocity information for the moving vehicle and the base stationlocation information, for use in determining a target base station insaid plurality to be used when performing a handover procedure totransition communication with the moving vehicle from a current basestation in said plurality to the target base station.

By obtaining base station location information for the base stations,and current position and velocity information for the moving vehicle,the handover metrics computation circuitry is able to generate one ormore handover metrics that can take into account the separation betweenthe moving vehicle and the various base stations, and also take intoaccount factors that may arise from the relatively high speed of themoving vehicle, with those generated metrics then being taken intoaccount when determining a suitable target base station to transitioncommunication with the moving vehicle to during a handover procedure.

The above described apparatus can be located at a variety of locationswithin the system. For example, the apparatus could be provided withinthe moving vehicle, in order to generate handover metrics specific tothat moving vehicle. However, due to the nature of the information usedin computing the above described handover metrics, in one exampleimplementation the apparatus can be provided as a centralised handoveranalysis system to perform the generation of handover metrics for anymoving vehicles connected to the wireless network. The handover metricsdescribed herein can be computed without needing to receive any signalquality measurements from the moving vehicles, and the generatedhandover metrics can be used to enhance the algorithms used to evaluatethe decision to trigger a handover from one base station to another basestation.

In one example implementation, the handover metrics computationcircuitry is arranged to generate the at least one handover metric foreach of a plurality of candidate target base stations. The candidatetarget base stations can be chosen in a variety of ways. For example,they may typically be determined with reference to the current basestation (i.e. the base station that the moving vehicle is currentlyconnected to), and hence for instance a network neighbourhood may beidentified based on the current base station, for example to identify aplurality of candidate target base stations within a certain range ofthe current base station. If desired, the choice of candidate targetbase stations can also be influenced by other factors. For example, thedirection of travel and/or bearing of the moving vehicle can be used inorder to identify the track that the moving vehicle is taking across thearrangement of base stations used to provide the wireless network and/orthe orientation of the moving vehicle's antennas, and to thuspotentially eliminate some of the base stations in the networkneighbourhood based on that information.

As mentioned earlier, a number of technical issues can arise whenseeking to support modern telecommunications Standards such as 4G (LTE)in systems such as ATG systems. One particular issue that arises isinterference between carrier signals due to the impact of the Dopplereffect on the frequencies of signals transmitted between the basestation (also referred to herein as the ground terminal) and the movingvehicle (for example an aircraft). This is particularly significant inmodern telecommunications Standards such a 4G, due to the high frequencyof signals that are transmitted according to these Standards—coupledwith the high speeds with which moving vehicles such as modernaeroplanes travel, this means that the Doppler effect can be significantin ATG systems, since the Doppler effect is dependent on both thevelocity of the vehicle and the frequency of the signal.

While it may be possible to mitigate some of the problems caused by theDoppler effect by choosing modulation schemes for the signals that aremore resilient to interference, such schemes typically result in reducedthroughput, which has the unwanted effect of lowering the capacity ofcommunication in the system. As described herein techniques can beadopted to seek to overcome some of the issues related to the Dopplereffect, without significantly reducing the capacity.

Considering the handover metrics computed by the handover metricscomputation circuitry of the apparatus described herein, then at leastone handover metric may comprise at least one Doppler effect metricindicative of the impact the Doppler effect will have on transmittedsignals between the moving vehicle and a candidate target base station.The handover metrics computation circuitry may then be arranged tocompute, for each of the plurality of candidate target base stations,said at least one Doppler effect metric for that candidate target basestation. Such Doppler effect metrics can then be taken into account whenmaking a handover determination, for example to seek to increase aprobability of selecting, as the target base station, a candidate targetbase station less affected by the Doppler effect.

The Doppler effect metric determined by the handover metrics computationcircuitry can take a variety of forms. In one example implementation,the at least one Doppler effect metric is an indication of relativespeed between the moving vehicle and the candidate target base station.In particular, as the relative speed increases between the movingvehicle and the candidate target base station, the Doppler effect willbecome more pronounced.

There are a number of ways in which the relative speed can be computedby the handover metrics computation circuitry, but in one examplearrangement the handover metrics computation circuitry is arranged tocompute the indication of relative speed between the moving vehicle andthe candidate target base station by computing, based on currentposition information for the moving vehicle and location information forthe candidate target base station, a separation vector extending betweenthe moving vehicle and the candidate target base station, and thencomputing a component of the velocity of the moving vehicle along thatseparation vector. In one example implementation, the computed relativespeed may have a positive or a negative value, hence identifying whetherthe moving vehicle is moving closer to the candidate target base stationor moving further away from the candidate target base station. Whilstthis signed relative speed could be used as the Doppler effect metric,in one example implementation it is the magnitude of the relative speedthat is used as the Doppler effect metric, and hence the sign can beignored.

The handover metrics computation circuitry can generate one or moreDoppler effect metrics for each candidate target base station. Asanother example of a Doppler effect metric that can be generated, thehandover metrics computation circuitry may generate an indication of aspeed of change of the Doppler effect on transmitted signals between themoving vehicle and the candidate target base station. The more rapidlythe Doppler effect is changing, then the more rapidly the compensationapplied to the communications to try and eliminate the Doppler effectmay need to be adjusted, and it may hence become more challenging toseek to eliminate the Doppler effect when the Doppler effect is changingrapidly. Hence it can be useful to provide an indication of the speed ofchange of the Doppler effect, so that for example preference can begiven to selecting a target base station exhibiting a lower speed ofchange of the Doppler effect.

In one example implementation, the handover metrics computationcircuitry is arranged to compute the indication of a speed of change ofthe Doppler effect on transmitted signals between the moving vehicle andthe candidate target base station by computing a change in relativespeed between the moving vehicle and the candidate target base station.

Another issue that can arise in systems such as ATG systems, due to therelatively large distance between the base stations, is that timingadvances may be required in respect of certain signals communicated bythe moving vehicle to the connected base station in order to ensure thatthose communications are received at the base station at an expectedtiming. In particular the delay in transmission caused by the separationdistance between the moving vehicle and the base station may need to betaken into account when computing the timing of transmission of certainsignals.

However, the antenna system in the moving vehicle may be arranged sothat it can only transmit or receive at any point in time. Accordingly,it cannot simultaneously receive downlink communications from the basestation to the moving vehicle whilst also transmitting uplink signalsfrom the moving vehicle to the base station. A number of sub-frames maybe allocated for transmission of uplink signals, but as the timingadvance required increases, one or more of the uplink sub-frames may nolonger be able to be used, and this can affect uplink capacity. In oneexample implementation, the handover metrics computation circuitry seeksto take this issue into account by generating an appropriate handovermetric. In particular, the at least one handover metric may comprise anuplink capacity metric indicative of a number of sub-frames availablefor allocation for uplink communication from the moving vehicle to thecandidate target base station, and the handover metrics computationcircuitry may be arranged to compute, for each of the plurality ofcandidate target base stations, the uplink capacity metric for thatcandidate target base station.

In one example arrangement, the number of sub-frames available forallocation for uplink communication from the moving vehicle to thecandidate target base station is dependent on a separation distancebetween the moving vehicle and the candidate target base station. Thehandover metrics computation circuitry may then be arranged to computethe uplink capacity metric for each candidate target base station bycomputing, based on current position information for the moving vehicleand location information for the candidate target base station, aseparation distance between the moving vehicle and the candidate targetbase station, and determining the uplink capacity metric in dependenceon the computed separation distance.

Whilst the uplink capacity metric may be determined based solely on thenumber of sub-frames available for allocation for uplink communication,if desired additional information can also be incorporated into thedetermination of the uplink capacity metric. For example, in oneimplementation the handover metrics computation circuitry may bearranged to receive capacity reports from the candidate target basestations about uplink capacity utilisation, and to determine the uplinkcapacity metric in dependence on both the computed separation distanceand the capacity report received from the candidate target base station.The capacity reports can provide information about the uplink capacityutilisation in a variety of ways. For example, they may identify unuseduplink capacity, i.e. spare uplink capacity. This information could becombined with the information indicative of the number of sub-framesavailable for uplink communication, for example to produce an effectiveuplink capacity metric normalised to the same capacity units, e.g.resource blocks, where a resource block is the smallest allocableportion of the communication frame.

By generating an uplink capacity metric as discussed above, this canthen be used to increase a probability of selecting, as the target basestation, the candidate target base station having the most uplinkcapacity.

In addition to, or as an alternative to, the above described handovermetrics, one or more further handover metrics can also be computed bythe handover metrics computation circuitry. For example, the movingvehicle tracking circuitry may further be arranged to obtain a bearingindication for the moving vehicle, and the handover metrics computationcircuitry may be provided with antenna information for the movingvehicle. The antenna information can take a variety of forms, but mayfor example identify the layout of antennas and the beam patterns beingused by those antennas within the moving vehicle. In combination withthe bearing information, the geographic coverage of the beam patternsused by the moving vehicle can be determined. The handover metricscomputation circuitry may then be arranged to compute, as at least oneof the handover metrics, an antenna gain metric. In particular, thehandover metrics computation circuitry may be arranged to compute, foreach of the plurality of candidate target base stations, the antennagain metric based on the antenna information, the bearing of the movingvehicle and a separation vector extending between the moving vehicle andthe candidate target base station, where the separation vector iscomputed from current position information for the moving vehicle andlocation information for the candidate target base station.

The production of such a metric can be very useful, as for example itmay be the case that a base station that is relatively close to themoving vehicle is not in fact a good candidate target base station, dueto there being a poor antenna gain metric for that base station.

By generating an antenna gain metric in the manner discussed above, thisenables an estimation of signal receive power to be produced withoutneeding to receive any measurement reports from the moving vehicleitself, and hence enables an estimate of signal quality to be obtainedwithout for example receiving the earlier discussed RSRP measurement.

In one example implementation, the handover metrics computationcircuitry not only generates the at least one handover metric for eachof the plurality of candidate target base stations but also generatesthe at least one handover metric for the current base station. Thishence enables an evaluation as to whether a handover from the currentbase station is in fact appropriate, or whether for the time beingcommunication should be maintained with the current base station.

In one example arrangement the apparatus further comprises handoverdecision circuitry to select, based on the handover metrics generated bythe handover metrics computation circuitry, which one of the pluralityof candidate target base stations is to be used as the target basestation to which communication with the moving vehicle is to betransitioned from the current base station. Further, as discussedearlier, with the provision of equivalent handover metrics for thecurrent base station, the handover decision circuitry can also determinewhether to transition communication with the moving vehicle from thecurrent base station to one of the plurality of candidate target basestations, or instead maintain communication with the current basestation.

In one example implementation, the handover metrics generated by thehandover metrics computation circuitry are subjected to a normalisationand weighting operation prior to being used by the handover decisioncircuitry. By normalising the handover metrics, the different handovermetrics can be represented on the same scale, and then weighting can beused to attribute greater importance to some handover metrics thanothers, thus enabling configurability as to the influence the varioushandover metrics have on the handover decision made by the handoverdecision circuitry.

In one example implementation, there may be a configurable range ofvalid values for the various handover metrics, and if any particularhandover metric falls outside of that configurable range, this mayindicate that the associated candidate target base station is not a goodcandidate target base station to consider when making the handoverdecision. Accordingly, in one example implementation, the handovermetrics computation circuitry may be arranged to perform a filteringoperation using the generated handover metrics in order to determinewhether to eliminate any of the plurality of candidate target basestations from consideration by the handover decision circuitry. Hencepurely by way of example, if a very poor antenna gain is associated witha particular candidate target base station, it may be decided toeliminate that candidate target base station from the group of candidatetarget base stations considered by the handover decision circuitry. Thehandover decision circuitry would then only consider the handovermetrics generated for the remaining candidate target base stations.

In a typical 4G (LTE) system, the currently connected base station wouldcontrol the handover process, based for example on the RSRP and RSRQmeasurements received from the moving vehicle. Whilst in accordance withthe techniques described herein the base station could still be involvedin the handover decision process, for example by taking into account thehandover metrics computed by the handover metrics computation circuitry,in one example implementation a forced handover process is insteadimplemented. In particular, in one example arrangement, the handoverdecision circuitry is arranged to issue a forced handover signal to thecurrent base station, identifying the target base station, in order toinitiate performance of a handover procedure to transition communicationwith the moving vehicle from the current base station to the target basestation. Hence, in such an arrangement, the handover decision circuitrycan issue a handover request signal to the currently connected basestation, in order to cause that currently connected base station toinitiate performance of a handover procedure to transition communicationto the identified target base station. This can provide an efficientmechanism for controlling handover using the metrics generated by thehandover metrics computation circuitry.

Particular examples will now be described with reference to the Figures.

The moving vehicles for which the techniques described herein can beutilised can take a variety of forms. For instance, the techniques couldbe applied in respect of trains, where the ground terminals may bespread out along the track. However, for the purposes of the examplesdiscussed herein, it will be assumed that the moving vehicle is anaircraft, such as the airplane 10 shown in FIG. 1. As shown in FIG. 1,the airplane 10 is able to communicate with a ground terminal 20 (whichmay also be referred to herein as a ground station). A network of groundterminals will be provided, enabling the aircraft 10 to connect todifferent ground terminals during a flight in order to seek to maintaina communication link that can be used to provide connectivity topassengers in the aircraft. As shown in FIG. 1, the aircraft 10 isassumed to be travelling at a velocity 40, and has a relative separation30 between it and the ground terminal that it is connected to. Thisrelative separation can be specified as a vector, as can the velocity40, and there will be an angular separation between the velocity vectorand the relative separation vector, namely the angle 50 shown in FIG. 1.

Communication between the aircraft 10 and a ground station 20 with whicha communication link is established can take place within communicationframes. An example communication frame that may be used is illustratedin FIG. 2. Here, the communication frame 60 is defined in both thefrequency and time domains. In particular, in the time domain, the framecan be considered as consisting of a plurality of sub-frames 70. In oneparticular example, a communication frame 60 is 10 milliseconds (ms)long, and there are ten sub-frames in the communication frame, whereeach sub-frame has a duration of 1 ms. Each sub-frame 70 comprises anumber of resource blocks (the resource blocks not being shownseparately in FIG. 2), a resource block being the smallest allocableportion of the communication frame.

A sub-frame may be allocated for downlink communication (also referredto herein as forward link communication) from a ground terminal 20 tothe aircraft 10, or can be allocated for uplink communication (alsoreferred to herein as reverse link communication) from the aircraft 10to the ground terminal 20. In FIG. 2, sub-frames allocated for downlinkcommunication are prefixed with the letter “D” and sub-frames allocatedfor uplink communication are prefixed with the letter “U”. As also shownin FIG. 2, one or more sub-frames may be allocated as special sub-frames(prefixed by the letter “S”). These can be used as a gap sub-frame toprovide some separation between downlink communication and uplinkcommunication. However, it is possible that not the entirety of thespecial sub-frame is left as a gap. In particular, each sub-frame can beconsidered as consisting of a plurality of symbols, in one particularexample there being 14 symbols within each sub-frame. Hence, one or moreof the symbols may be allocated for downlink communication and one ormore of the symbols may be allocated for uplink communication, with theremaining symbols being left free. In one specific implementation of thecommunication frame format shown in FIG. 2, the first three symbolswithin the special sub-frame S0 can be used for downlink communication,and the final symbol may be used for uplink communication. This leaves10 symbols free, which in one embodiment equates to a 0.712 ms gap.

FIG. 3 is a block diagram illustrating more details of the componentsprovided within a vehicle terminal 100 and a ground terminal 130. Thevehicle terminal 100 may for example be provided within the aircraft 10shown in FIG. 1, whereas the ground terminal 130 may form the groundstation 20 shown in FIG. 1.

The vehicle terminal 100 has an antenna system 105 used to communicatewirelessly with the ground terminal 130. The antenna system 105 mayinclude all of the electronics used to convert between baseband and RFsignals for both data to be transmitted from the vehicle terminal'santenna and for data received by the vehicle terminal's antenna.Communication control circuitry 110 is provided for controlling theoperation of the antenna system 105. To assist the communication controlcircuitry 110 in performing the control operations to be described inmore detail herein, the communication control circuitry 110 has accessto distance computation circuitry 120 that can be used to determine theseparation between the vehicle terminal 100 and the ground terminal 130.In some example implementations, that separation is expressed as avector identifying the relative separation between the two antennasystems, whilst in other implementations that separation may beexpressed as an absolute separation distance (i.e. a scalar term ratherthan a vector term).

The distance computation circuitry 120 may have access to locationspecifying circuitry 115 that can provide information identifying thecurrent location of the vehicle terminal 100. The location specifyingcircuitry can take a variety of forms, but in one example implementationis a GPS receiver.

The distance computation circuitry 120 can be arranged to operate in avariety of ways, but in one example implementation extracts informationfrom a downlink communication in order to seek to identify the locationof the ground terminal 130. That information could in principle directlyidentify the coordinates of the ground terminal, but in one exampleimplementation that information is an identifier of the ground terminal,and the distance computation circuitry uses that identifier in order toobtain the coordinates of the ground terminal.

In particular, as shown in FIG. 3, in one example implementation thevehicle terminal 100 has a storage device 125 providing a correlationbetween ground terminal identifiers and associated location information.Accordingly, a lookup operation can be performed within the storageusing the identifier information extracted from the downlink signal, inorder to obtain the location information of the ground terminal. Usingthat information, and the location information from the GPS receiver115, the distance computation circuitry 120 can then calculate theseparation between the vehicle terminal and the ground terminal.

As shown in FIG. 3, the ground terminal will include a further antennasystem 135, which is controlled by communication link establishing andscheduling circuitry 140. The functionality performed by thecommunication link establishing and scheduling circuitry 140 will bediscussed in more detail later. However, in one implementation thatcomponent has access to distance computation circuitry 145 that cancompute the separation between the ground terminal 130 and the vehicleterminal 100. As with the earlier-discussed distance computationcircuitry 120, the distance computation circuitry may produce thatseparation as a vector quantity, or as a scalar quantity dependent onimplementation. In one example implementation, the distance computationcircuitry will know the coordinate information of the ground terminal130, which it will be appreciated is fixed, and will obtain vehicletracking information indicative of the current location of the vehicleterminal 100. This vehicle tracking information can be obtained in avariety of ways. However, considering the example of an aircraft 10shown in FIG. 1, it will be appreciated that there are availableresources that track in real time the coordinates of aircrafts, and thatinformation can be obtained in order to provide the distance computationcircuitry 145 with the required vehicle tracking information for thevehicle terminal 100.

The separation between the ground terminal 130 and the vehicle terminal100 determined by the distance computation circuitry 120 is calculatedas a vector value, indicating both a magnitude (distance) and direction(angle). In one example implementation, analysis circuitry performs aDoppler adjustment process to determine an adjustment to be made to thetransmission frequency of the uplink (reverse link) signal, based on thevector separation determined by the distance computation circuitry. Theanalysis circuitry therefore encompasses the distance computationcircuitry 120 and at least some of the functionality of thecommunication control circuitry 110. The transmitted frequency (f_(t))of the transmitted signal (uplink signal) is determined such that theobserved frequency of the uplink signal when it is received by thefurther antenna system 135 equals a predetermined uplink frequency(f_(UL)); this is the frequency at which the ground terminal 130 expectsto receive the uplink signal, corrected (by the Doppler adjustmentprocess) to account for the Doppler effect in both the received andtransmitted signals. The Doppler adjustment process is described in moredetail with reference to the examples given below.

FIG. 4 is a flow diagram illustrating a method of operation of thevehicle terminal 100. In a first step S402, a received signal (thedownlink/forward link signal) is received at the antenna of the antennasystem 105, the received signal having a received frequency (f_(r)). Atleast one item of information—for example, information with which thedistance computation circuitry 120 can determine the vector separationbetween the antenna system 105 of the vehicle terminal 100 and thefurther antenna system 135 of the ground terminal 130—is obtained atstep S404 from the received signal by the distance computation circuitry120. The information is then used in a Doppler adjustment process S406,to determine the transmitted frequency (f_(t)) with which the uplinksignal is to be transmitted, taking into account any frequency shiftsdue to the Doppler effect.

Once the Doppler adjustment process S406 has been performed, then atstep S408 the antenna system 105 can transmit, at the transmittedfrequency (f_(t)), the uplink signal to the further antenna system 135.

FIG. 5 is a flow diagram showing an example of the Doppler adjustmentprocess S406 a referred to in FIG. 4. This particular example refers tothe case where the information extracted from the received signal is anidentifier of the ground terminal 130.

The Doppler adjustment process of this example starts at a step S502. Atstep S504 the distance computation circuitry 120 obtains, from thereceived downlink signal, an identifier of the ground terminal 135.Using this identifier, the computation circuitry 120 can then index thestorage structure 125 in order to determine at step S506 the location ofthe ground station. The location of the vehicle terminal, along with itsvelocity, are also determined at step S508. At least the location can bedetermined by the location specifying circuitry 115, but in instanceswhere the location specifying circuitry 115 is a GPS receiver it will beappreciated that the velocity information can also be determined fromthe output of the GPS receiver. Using the locations of the groundterminal 130 and the vehicle terminal 100, the vector displacement(separation) between the two terminals can be determined at step S510 bythe distance computation circuitry 120, and thus an adjustment value(Δf) representative of the change in frequency of the received signaldue to the Doppler effect can be calculated at step S512. Thiscalculation is performed by the analysis circuitry according to theDoppler formula:

${\Delta f} = {\frac{r \cdot v}{{❘r❘}c}f_{DL}}$

where r is the vector separation between the ground terminal 130 and thevehicle terminal 100, v is the velocity of the vehicle terminal 100, cis the speed of light and f_(DL) is the predetermined downlink frequency(the frequency at which the ground terminal 130 transmits the downlinksignal).

This adjustment value (Δf) is then used to calculate the transmittedfrequency (f_(t)) with which the uplink signal is to be transmitted, inaccordance with the following formula:

f _(t) =f _(s)−2Δf

where f_(r) is the received frequency of the downlink signal. The abovecalculations assume that a time division duplex (TDD) scheme isemployed, in which the predetermined uplink frequency and thepredetermined downlink frequency (the frequencies of the uplink anddownlink signals at the ground terminal) are the same. The receivedfrequency of the downlink signal is f_(r)=f_(DL)+Δf, and that receivedfrequency is used as the default frequency for transmission from thevehicle terminal 100. Hence the frequency of the transmitted signalneeds to be adjusted by −2Δf in order to compensate for the Dopplereffect in both the received and transmitted signals, such that thefrequency of the uplink signal as observed by the ground terminal isf_(UL)=f_(DL).

However, the above approach can also be generalised to a frequencydivision duplex (FDD) scheme where the predetermined uplink and downlinkfrequencies differ, as discussed below with reference to FIG. 9, and theadjustment required to the default transmission frequency in that caseis the same.

While the example described with reference to FIG. 5 assumes that anidentifier of the ground station 130 is obtained from the downlinksignal, it is also possible for the downlink signal itself to specifythe location (e.g. the coordinates) of the ground terminal 130. In thiscase, steps S504 and S506 in FIG. 5 would be replaced with a single stepof obtaining, from the received signal, the location of the groundterminal 130.

Furthermore, in some examples it may also be possible to calculate theDoppler adjustment Δf without knowing the magnitude of the distance (r)between the two terminals, provided that at least the angle θ betweenthe vehicle's velocity and a line connecting the two terminals is known.This is because the dot product between r and v can be calculated as|r|*|v|*cos θ, so that cancels out in the Doppler formula. The angle θcould be calculated in any of a number of ways; for example, the angleof arrival (AoA) of the incoming downlink signal could be determinedusing a phase array, to determine the angle relative to the vehicle'sheading.

The examples described so far involve calculating, with distancecomputation circuitry 120, the vector separation between the groundterminal 130 and the vehicle terminal 100. However, other examplesinstead perform the Doppler adjustment process using information aboutthe received signal itself, rather than information about the groundterminal 130 (such as its location). One such example is demonstratedschematically in FIG. 6. A ground station 130 and an air station 600 (anexample of the vehicle terminal 100 shown in FIG. 3) are shown in FIG.6. The ground station 130 transmits a downlink signal 602 at a frequency(the predetermined downlink frequency f_(DL)) of 2.4 GHz (2.4 billioncycles per second). This signal is received a short time later at theair station 600, which is moving away from the ground station 130 with agiven velocity (v). Due to the Doppler effect, the frequency of thesignal as observed by the air station 600 is lower than 2.4 GHz (orhigher if the air station 600 is moving towards the ground station 100),meaning that the number of cycles per second has reduced. In thisexample, the frequency (f_(r)) of the downlink signal 602 as observed bythe air station 600 can be compared with the expected value of thefrequency (2.4 GHz) to determine an adjustment (Δf) to be applied to thetransmitted signal (not shown).

The air station 600 also receives a timing signal 604 from a GPSsatellite 606, which provides accurate timing information. This timinginformation can then be used by the air station 600 (more particularly,by the analysis circuitry in the air station 600) to accurately countthe number of cycles per second in the received signal 602, to determinehow the frequency has changed. This information can then be used by theanalysis circuitry to determine the transmitted frequency (f_(t)) of thetransmitted signal. Thus, FIG. 6 is an example of the use of informationrelating to the received signal itself in performing a Doppleradjustment process.

While the arrangement shown in FIG. 6 calculates the received frequency(f_(r)) of the downlink signal as the information relating to thereceived signal, there are other examples of information about thereceived signal that could be used instead, for example the number ofcommunication frames 60 received at the air station 600 per second(which can be compared to the expected value of 100 per second), thenumber of OFDM (Orthogonal Frequency Division Multiplexing) symbolsreceived per second, or the number of primary synchronisation signals(PSSs) counted per second. In fact, any property of the received signalthat is affected by the Doppler effect (so any property related to thefrequency of the signal) can be used.

FIG. 7 is a flow diagram illustrating another example of the Doppleradjustment process S406 b applied in FIG. 4, this time using informationrelating to the received signal, rather than information about theground terminal 130. In the following example, it is assumed that a TDDscheme is employed, and that the predetermined uplink frequency andpredetermined downlink frequency are, therefore, the same.

In FIG. 7, the process begins at a first step S702, before passing to astep S704 of obtaining, from the received signal, information relatingto the received signal itself. As mentioned above, this could includethe received frequency (f_(r)) of the received signal, or any otherproperty of the received signal impacted by the Doppler effect.

The obtained information is compared at step S706 with one or moreexpected values, allowing an indication of the Doppler effect on thereceived signal to be determined, and thus an adjusted transmissionfrequency (f_(t)) to be determined at step S708. Then, the antennasystem 105 transmits the adjusted transmitted signal with transmissionfrequency (f_(t)).

FIGS. 8 and 9 show in more detail some of the elements that may bepresent in the antenna system 105 and communication control circuitry110 of the vehicle terminal 100, in accordance with the exampledescribed with reference to FIG. 7; in particular, FIGS. 8 and 9describe components to be used in a system in which an indication of thereceived frequency (f_(r)) of the received signal is used to perform theDoppler adjustment process. FIG. 8 shows elements present in a vehicleterminal 100 to be used in a time division duplex (TDD) scheme, in whichthe predetermined uplink frequency and predetermined downlink frequencyare the same while FIG. 9 shows an alternative arrangement to be used ina frequency division duplex (FDD) scheme, in which the predetermineduplink frequency and predetermined downlink frequency may be different.As noted above, the predetermined downlink frequency is the frequencywith which the ground terminal transmits the downlink signal, and thepredetermined uplink frequency is the frequency at which the groundterminal expects to receive the uplink signal.

In FIG. 8, the received signal Rx is received at an antenna 800, thereceived signal having a frequency equal to the carrier frequency(f_(c)) (the downlink frequency) adjusted according to the Dopplereffect (f_(c)+Δf). The received signal is fed into a frequency mixer802, the output of which is fed into a low pass filter (LPF) 804. Thefrequency estimator 806 estimates the frequency of the received signalbased on the output of the LPF 804, and supplies the signal to abaseband receiver 808. The frequency estimator 806 also supplies acontrol voltage to a reference oscillator 810, to cause the referenceoscillator 810 to then output a reference signal at a frequency(F_(ref)+ΔF_(ref)). The reference signal is fed into a local oscillator812, which multiplies the reference frequency by an upscaling factor αand outputs the resulting signal—corresponding to an estimation of thefrequency of the received signal—back into the frequency mixer 802. Theupscaling factor α is determined based on an RF upconversion controlsignal received at the local oscillator 812 from an RF controller 814.Thus, the above process implements a feedback loop, and the frequencyestimated by the frequency estimator 806 is more accurate with everypass.

The signal output by the reference oscillator 810 is also fed into acounter 816. A timing signal, received at a GPS antenna 818 andprocessed by a GPS element 820 is also fed into the counter 816. Thetiming signal provides one pulse per second (PPS), and hence, using thetiming signal, the counter 816 can count the number of cycles per secondin the reference signal output by the reference oscillator 810.

The counter 816 feeds into a logic circuit 822, controlled by the RFcontroller 814, which determines a downscaled adjustment value(2Δf_(ref)). The downscaled adjustment value (2Δf_(ref)) and the outputof a baseband transmitter 826 (having a frequency of F_(s)) are then fedinto a second frequency mixer 824.

The second frequency mixer 824 then outputs a signal (F_(s)−2Δf_(ref))to a third frequency mixer 828. The third frequency mixer 828 alsoreceives an input from the local oscillator 812 (i.e. a signalrepresenting the received frequency), and outputs a signal withfrequency F_(c)−2Δf, which is the adjusted transmitted frequencydescribed in earlier figures. This signal can then be transmitted as theuplink signal by an antenna 830.

The arrangement shown in FIG. 9 is almost identical to that shown inFIG. 8, with one main difference: the arrangement in FIG. 9 alsoincludes a second local oscillator 902, which receives signals from thereference oscillator 810 and the RF controller 814. This allows for thesignal fed into the third frequency mixer 828 (F_(c_ul)+Δf__(dl)) totake into account the difference in frequency between the uplink anddownlink signals in an FDD system.

It should be noted that the frequency of the transmitted signal isadjusted by a value of 2Δf, regardless of whether or not the downlinkfrequency f_(DL) and the uplink frequency f_(UL) are the same. This canbe shown as follows:

The Doppler frequency on the Forward Link (FL) is given by:

${\Delta f^{FL}} = {\frac{r \cdot v}{{❘r❘}c}f_{c}^{FL}}$

where f_(c) ^(VL) denotes the centre frequency on the forward link(downlink), c is the speed of light, v is the velocity vector and r isthe relative distance to the base station. The (⋅) symbol denotes thedot product operator, whereinr·v=(r_(x),r_(y),r_(z))·(v_(x),v_(y),v_(z))=r_(x)v_(x)+r_(y)v_(y)+r_(z)y_(z).

The Doppler frequency on the Reverse Link (RL), assuming that thecarrier frequency is f_(c) ^(EL), is given by:

${\Delta f^{RL}} = {\frac{r \cdot v}{{❘r❘}c}f_{c}^{RL}}$

The reference oscillator will therefore converge to:

$f^{REF} = {{( {f_{c}^{FL} + {\Delta f^{FL}}} )/a^{FL}} = {( {1 + \frac{r \cdot v}{{❘r❘}c}} ){f_{c}^{FL}/a^{FL}}}}$

where α^(EL) denotes the upscaling (multiplicative) factor for theforward link. For example, if f^(REF)=40 MHz, the α^(FL)=60 to ensurethe centre frequency will be at 2.4 GHz.

The received frequency at the base station (ground terminal) will bemultiple of the reference frequency (f^(REF)α^(RL)), adjusted by theDoppler effect. That is

${f^{{RX} - {RL}} = {{( {1 + \frac{r \cdot v}{{❘r❘}c}} )f^{REF}a^{RL}} = {( {1 + \frac{r \cdot v}{{❘r❘}c}} )( {1 + \frac{r \cdot v}{{❘r❘}c}} )}}},{c_{c}^{FL}\frac{a^{RL}}{a^{FL}}}$$= {{( {1 + {2\frac{r \cdot v}{{❘r❘}c}} + ( \frac{r \cdot v}{{❘r❘}c} )^{2}} )f_{c}^{FL}\frac{a^{RL}}{a^{FL}}} \approx {( {1 + {2\frac{r \cdot v}{{❘r❘}c}}} )f_{c}^{FL}\frac{a^{RL}}{a^{FL}}}}$$( \frac{r \cdot v}{{❘r❘}c} )^{2} \approx {0{since}v^{2}{{\operatorname{<<}c^{2}}.}}$

To prove this assumption, assuming 1000 km/h at 2.4 GHz,

${( \frac{r \cdot v}{{❘r❘}c} )^{2}f_{c}} = {0.002{{Hz}.}}$

this is an insignificant contribution and can be ignored.

In TDD (time division duplex), α^(FL)=α^(RL) and f_(c) ^(FL)=f_(c)^(RL)=f_(c), which implies that Δf^(FL)=Δf^(RL)=Δf, and thusf^(RX-BV)=f_(c)+2Δf.

Note that in FDD (frequency division duplex) (or TDD),

${f_{c}^{RL} = {f_{c}^{FL}\frac{a^{FL}}{c^{FL}}}},$

thus

$f^{{RX} - {RL}} = {{( {1 + {2\frac{r \cdot v}{{❘r❘}c}}} )f_{c}^{RL}} = {f_{c}^{RL} + {2\Delta f^{RL}}}}$

Therefore, just like in the TDD case we need to compensate thetransmission by 2Δf^(RL).

As shown through the above examples, the present technique allows thefrequency of a signal transmitted by a wireless communication systeminstalled in a fast-moving vehicle to be adjusted to compensate for theDoppler effect. This reduces interference effects at a ground terminal(base station), and allows higher frequency signals (such as those usedin modern telecommunications Standards) to be used. It also allows thesystem to be used in vehicles of increasing speeds. Thus, moderntelecommunications Standards such as 4G (LTE) can be implemented in ATGsystems, even as the speeds with which modern aeroplanes travel areever-increasing.

One of the functions performed by the communication control circuitry110 is to perform a sign-on procedure to seek to establish acommunication link with the ground terminal 130. During that sign-onprocedure, the communication control circuitry 110 will issue aconnection setup signal for receipt by the further antenna system 135within an identified timing window. The vehicle terminal 100 willfirstly receive an initial signal from the ground terminal 130 advisingof the availability for the connection setup signal to be issued, andproviding information regarding the identified timing window. The timingwindow will typically occupy one or more sub-frames, and the connectionsetup signal will have a duration less than the identified timingwindow, but will need to be received in its entirety within that timingwindow in order for a connection to successfully be established.

In accordance with the techniques described herein, it is assumed thatcommunications are taking place in accordance with the 4G (LTE)Standard, and such a connection setup signal may be referred to as aRACH (random access channel) signal that is issued in a random accesschannel during an uplink communication from the moving vehicle to theground terminal. Different RACH configurations may be supported, forexample associated with different sized RACH signals and associateddifferent sized timing windows.

FIG. 10A illustrates an example form of RACH configuration that could beused when adopting the communication frame format of FIG. 2, and insituations where the separation between the aircraft 10 and the groundterminal 20 does not exceed 108 km. Here the timing window occupiesthree sub-frames. As indicated by the communication frame 1000, it isassumed that the ground station 20 transmits a signal identifying thatthere is a RACH opportunity that the aircraft can utilise in an uplinkcommunication back to the ground terminal 20. As shown by the line 1005in FIG. 10A, the receipt of the communication frame at the aircraft 10is delayed by approximately 0.33 ms, due to the separation between theaircraft and the ground terminal (in this case it being assumed thatthere is essentially the maximum separation that can be supported usingthis RACH format). As shown by the line 1010, it is assumed that theaircraft 10 then transmits the RACH signal, in this case the RACH signalbeing propagated across all three of the uplink communicationsub-frames.

It will be appreciated that that uplink transmission will also bedelayed by the same propagation delay, and hence will be received by theground terminal 20 at approximately 0.66 ms delay (as indicated by theline 1015), due to the round trip delay between the ground terminal andthe aircraft. However, the timing control at the ground terminal isfixed, and hence it will assume the timing of the sub-frames is alignedwith the initial timing shown by the entry 1000. Hence, it willinterpret the received information on that basis.

In this case it is assumed that the RACH signal is received entirelywithin the RACH timing window, and based on the relative offset of thatRACH signal, the ground station 20 can identify that the totalpropagation delay is 0.66 ms. Accordingly, in a subsequent communicationframe 1020 where the ground station provides a response to identify thata successful communication link has been established, that responsesignal from the ground station will identify that the aircraft shouldadvance its timing for subsequent uplink communication by 0.66 ms. As aresult, this will ensure that the subsequent uplink communication isaligned with the sub-frame timing boundaries as implemented by theground terminal 20.

FIG. 10B illustrates the use of the same example RACH configuration, butin a situation where the separation exceeds the maximum separationdistance of 108 km. In this specific example, it is assumed that theseparation is 150 km resulting in a 0.5 ms propagation delay from theground terminal 20 to the aircraft 10. As shown by the line 1025, theground terminal 20 emits the same initial signal as discussed earlierwith reference to the line 1000 of FIG. 10A, and hence identifies a RACHopportunity. However, as shown by the line 1030, the communication frameis received after a 0.5 ms propagation delay. Again, as indicated by theline 1035, the aircraft terminal transmits the RACH signal within theuplink sub-frames, but again the communication is delayed by another 0.5ms on its transit to the ground terminal. Hence, there has been anoverall delay of 1 ms, and this results in the RACH signal not fallingwithin the RACH timing window, when using the timing adopted by theground station 20, as indicated by the line 1040. Accordingly, asindicated by line 1045, the RACH signal has not been successfullyreceived, and the ground station 20 will not send a response to theaircraft, as a result of which a communication link will not beestablished.

In accordance with the techniques described herein, this problem isaddressed by enabling the vehicle terminal to assess the separationbetween it and the ground terminal with which it is seeking to establisha communication, and to apply an initial timing advance relative to thedefault time indicated for the RACH signal, when issuing that RACHsignal to the ground terminal. This can be used to ensure that the RACHsignal is received within the specified timing window, hence enabling asuccessful communication link to be established. This process isdiscussed in more detail with reference to the flow diagram of FIG. 11.

As shown in FIG. 11, at step 1050 the vehicle terminal 100 observes asignal from the ground terminal 130 advising of the availability for theissuance of a connection setup signal (a RACH signal). This informationreceived by the vehicle terminal 100 also provides information about thedefault timing for issuing the RACH signal, the format of the RACHsignal, and the format of the timing window.

At step 1055, the distance computation circuitry 120 obtains thelocation information for the ground terminal, and determines aseparation distance between the vehicle terminal and the groundterminal. As discussed earlier, the distance computation circuitry 120may refer to the storage 125 in order to obtain the coordinates of theground terminal, based on that ground terminal's identifier includedwithin the communication from the ground terminal, and can obtaininformation about the location of the vehicle terminal from the GPSreceiver 115, hence enabling the separation distance to be determined.

At step 1060, it is determined whether the separation distance exceeds asetup threshold distance. If it does not, then the process proceeds tostep 1065, where the connection setup signal is sent in the standardmanner at the default timing, as per the process discussed for exampleearlier with reference to FIG. 10A. The setup threshold distance willdepend on the RACH configuration used, i.e. the format of the RACHsignal, and the size of the timing window, and the setup thresholddistance will be determined not to have been exceeded if the separationdistance is such that the RACH signal will be successfully received bythe ground station if merely transmitted at the default timing specifiedby the signal received at step 1050.

However, if at step 1060 it is determined that the separation distanceexceeds the setup threshold distance, then at step 1070 an initialtiming advance is chosen based on that separation distance. There are anumber of ways in which that initial timing advance can be determined,and one approach will be discussed later with reference to FIG. 14.

Once the initial timing advance has been determined at step 1070 then atstep 1075 the RACH signal is sent in the RACH channel at a timing basedon the initial timing advance. In particular, the default time isadjusted by the initial timing advance so that the RACH signal is issuedahead of the default time.

Due to the way in which the timing advance is chosen at step 1070, itwill hence be ensured that the RACH signal will be received within theRACH timing window by the ground station 130 even though the separationdistance exceeds the setup threshold distance.

Following either step 1065 or step 1075, the process proceeds to step1080, where the vehicle terminal 100 waits to see if a response isreceived from the ground terminal before a timeout period has elapsed.In particular, even though the RACH signal will have been receivedwithin the required timing window, it is not guaranteed that the groundterminal will choose to establish a communication link with the vehicleterminal. For example, it may be that the vehicle terminal is contendingwith a number of other vehicle terminals to establish a communicationlink, and the ground terminal may choose to establish a communicationlink with one or more of those other vehicle terminals instead of thecurrent vehicle terminal. For instance, certain vehicle terminals may begiven priority over others, and hence it may be that the vehicleterminal being considered in FIG. 11 does not obtain a communicationlink at that time.

If the ground terminal chooses not to establish a communication link, itwill not send a response back to the vehicle terminal, and accordinglyif such a response is not received within a certain timeout period, theprocess proceeds to step 1090 where the vehicle terminal will wait toretry establishing a communication link.

It may be that at step 1090 the vehicle terminal waits for another RACHopportunity to be identified by the same ground terminal, and thenretries establishing a communication link with that ground terminal. Itcould at that time take certain steps to increase the likelihood of itbeing allocated a communication link, such as for example increasing thepower of the transmission so as to indicate to the ground terminal thata better quality communication link could be established. For example,in one implementation, the vehicle terminal estimates path loss andcomputes an initial RACH power for detection, selects a preamble from anavailable set of preambles and transmits it. If that RACH request is notsuccessful, the vehicle terminal may autonomously choose another randompreamble and increase its power for the next RACH opportunity. This cancontinue until the vehicle terminal's maximum transmit power has beenreached.

However, the vehicle terminal is not limited to retrying to make aconnection with the same ground terminal, and if it receives an initialsignal from another ground terminal providing a connection setupopportunity, it could then seek to repeat the process of FIG. 11 inorder to establish a link with that ground terminal.

If at step 1080 it is determined that a RACH response is received fromthe ground terminal, hence identifying that the ground terminal hasaccepted the establishment of a communication link with the vehicleterminal, then the communication control circuitry 110 within thevehicle terminal 100 will analyse the response in order to determine howto control subsequent communication with the ground terminal. Inparticular, a further timing advance may be specified in the responsewhich should be used in combination with the initial (coarse) timingadvance chosen at step 1070 to control the timing of subsequent uplinkcommunication to the ground terminal. In addition, the response willtypically provide information about which sub-frames are allocated tothe vehicle terminal for downlink and uplink communications, so that thevehicle terminal can receive downlink communications destined for it asissued by the ground terminal 130, but can also issue its uplinkcommunications within an appropriate sub-frame, using the cumulativetiming advance determined at step 1085 so as to ensure that those uplinkcommunications are received at the appropriate timing by the groundterminal 130.

It should be noted that while the information in the RACH response isused to provide a fine timing advance that can be combined with thecoarse timing advance to determine the actual timing advance to be usedfor a subsequent uplink communication, as time progresses after thecommunication link has been established the distance between theaircraft and the ground terminal will change. This change can becompensated for using standard techniques provided by the 4G (LTE)Standard to make fine timing adjustments during the duration of thecommunications link.

FIG. 12A illustrates how the process of FIG. 11 is applied for aparticular implementation of the RACH signal and RACH timing window. Inthis example, it assumed that the RACH timing window is specified ascoinciding with the third uplink communication sub-frame (U2), and thatthe RACH signal as transmitted will need to land entirely within thatsub-frame in order for a successful communication to be established. Asindicated by the line 1100, the ground station transmits a signalidentifying the RACH opportunity that can be used within the uplinkpath. As indicated by the line 1105, due to the separation between theground terminal 130 and the vehicle terminal 100, which in this case isassumed to be the maximum allowable distance of 300 km, the vehicleterminal 100 receives the communication frame delayed by 1 ms, and hencethe communication frame is offset by a sub-frame width.

As indicated by the line 1110, because the separation distance exceedsthe setup threshold distance at step 1060, an initial timing advance ischosen at step 1070 based on the separation distance, and in this casethat initial timing advance will be chosen to be 2 ms. A full 2 msadvance can be applied without risk of violating a receive/transmittiming constraint, since even when the RACH signal is advanced by 2 ms,the vehicle terminal is not seeking to transmit that RACH signal at atime when it should be configured for receiving downlink communication,as is evident by the line 1110.

As indicated by the line 1115, that RACH signal will then actually bereceived with a 1 ms delay relative to its transmission time, which thenrealigns the RACH signal with the RACH timing window. Accordingly, theconnection setup signal (the RACH signal) will be received, andaccordingly a communication link can be established.

Assuming the ground terminal determines that a communication link is tobe established with the vehicle terminal, then it will transmit acommunication frame 1120 as a RACH response, which will be received witha 1 ms delay, as indicated by the line 1125. This can specify a finetiming advance if needed, which can be applied in combination with thecoarse timing advance applied by the vehicle terminal to controlsubsequent uplink communications. The RACH response will also typicallyprovide an indication of which sub-frames are allocated to the vehicleterminal for subsequent downlink and uplink communications.

As indicated in FIG. 12B, it is assumed in this instance that thevehicle terminal is allocated as its uplink sub-frame the sub-frame U2,and will accordingly perform an uplink transmission at a timingindicated by the line 1130 for its subsequent uplink communications. Asindicated by the line 1135 in FIG. 12B, due to the timing advanceapplied, this will ensure that the uplink communication is actuallyreceived at the correct timing by the ground terminal 130.

It should be noted that whilst in FIG. 12A it is assumed that the RACHconfiguration specifies that the RACH timing window is associated withthe U2 sub-frame, as discussed earlier different RACH configurations canbe used. For example, a RACH configuration may be used where the timingwindow is associated with both the U1 and the U2 sub-frames, with alonger RACH signal being issued, but with the requirement that a RACHsignal lands in its entirety within the U1 and U2 sub-frames as per thetiming adopted by the ground terminal 130. In another example, the RACHconfiguration may specify the use of all three uplink sub-frames as theRACH timing window, again with a longer RACH signal, but again with therequirement that that RACH signal lands entirely within the timingwindow as per the timing adopted by the ground terminal 130. The choiceof RACH configuration will affect the setup threshold distance that isassessed at step 1060 of FIG. 11, and may affect the initial timingadvance that is then chosen at step 1070 in situations where thedistance exceeds the setup threshold distance.

For instance, whilst in the example of FIG. 12A the initial timingadvance chosen based on the separation distance does not have to beconstrained to take into account the requirement not to violate areceive/transmit timing constraint, with other RACH configurations theinitial timing advance chosen may need to be constrained so as to ensurethat the receive/transmit timing constraint is not violated. Forexample, it will be appreciated that if the RACH timing window occupiesboth the U1 and the U2 sub-frames, and a 2 ms advance was applied as perthe example shown in FIG. 12A based on a separation distance of 300 km,this means that the transmission of the RACH signal will overlap withthe S0 sub-frame. However, the receive/transmit timing constraint wouldthen be violated if such an advance resulted in the need to transmit anuplink signal whilst the antenna system 105 should still be configuredfor downlink communication. In addition to the fact that it takes afinite time to perform the switch, as mentioned earlier it is alsopossible that some of the first symbols within the S0 sub-frame may beused for downlink communication, and accordingly in that instance it maynot be appropriate to fully advance the initial timing by the timingthat would be determined based purely on the propagation delay. Instead,it may be necessary to choose a slightly smaller coarse timing advanceto avoid violating the receive/transmit timing constraint, whilstensuring that that timing advance is sufficient to cause the RACH signalto be received within the RACH timing window. The further timing advancedetermined by the ground terminal will then compensate for the initialtiming advance, so that cumulatively the initial and further timingadvances will provide the required timing advance for subsequent uplinkcommunication.

FIG. 13 is a flow diagram illustrating one way in which step 1055 ofFIG. 11 may be performed. In this example, it is assumed that theinitial communication from the ground station includes a ground stationidentifier. At step 1150, the distance computation circuitry 120extracts that ground station identifier from the received signal, andthen at step 1155 performs a lookup in the database provided within thestorage 125 in order to obtain the location coordinates for the groundstation.

At step 1160, the distance computation circuitry 120 then obtainslocation coordinates of the vehicle terminal 100 from the GPS receiver115, and thereafter at step 1165 computes the separation distancebetween the ground terminal and the vehicle terminal.

Whilst the approach of FIG. 13 can be used in one exampleimplementation, in an alternative implementation it may be that theinitial signal from the ground terminal directly provided thecoordinates of the ground terminal, and accordingly those coordinatescould be extracted from the received signal at step 1150, and no lookupin the database would be required (hence step 1155 becoming redundant).

FIG. 14 is a flow diagram illustrating how step 1070 of FIG. 11 may beperformed in one example implementation. At step 1200, it is determinedwhich range of separation distances the separation distance fallswithin. Then, at step 1205 a timing advance appropriate for that rangeis determined. For instance, it could be that a lookup table is usedthat provides suitable coarse timing advances to be used for each of anumber of different ranges. That lookup table could provide timingadvances applicable for a number of different RACH configurations (i.e.for different formats of RACH signal and RACH timing window), with thelookup operation obtaining the timing advance appropriate for thedetermined range and RACH configuration.

However, in some implementations it may be determined that a lookuptable approach based on ranges is not required, and instead theseparation distance may be determined on the fly. In particular, aninitial timing advance can be determined by dividing the separationdistance by the speed of light.

As shown in FIG. 14, the process then proceeds to step 1210, where it isdetermined whether there is any receive/transmit timing violation issue.As discussed earlier, this may depend on the RACH configuration used andthe separation distance in question. In particular, for RACHconfigurations that use multiple sub-frames, it may be the case thatwhen the separation distance exceeds a certain amount, then there couldbe a receive/transmit timing violation issue if the timing advancedetermined at step 1205 was used “as is”.

If it is determined that there is not any receive/transmit timingviolation issue, then the process proceeds to step 1215 where thedetermined timing advance evaluated at step 1205 is used.

However, if it is determined that there is a receive/transmit timingviolation issue, then at step 1220 the timing advance can be scaled backto ensure that the receive/transmit timing constraint is not violated,whilst still enabling receipt of the connection setup signal within thetiming window.

In instances where the timing advance is encoded within a lookup tablebased on ranges of separation distance, then as mentioned earlier in oneexample implementation that lookup table will provide timing advanceinformation for each of a number of different possible RACHconfigurations, and the prospect of violating receive/transmit timingconstraints can be taken into account when populating the lookup table,so that in effect the evaluation at step 1210 is taken into account wheninitially populating the lookup table. In that event it will merely besufficient to determine the range that the separation distance fallswithin and then obtain the appropriate timing advance to use from thelookup table at step 1205. Hence, in that case steps 1210, 1215 and 1210would not be needed.

In one example implementation, when determining the appropriate timingadvance to use, the aim is to try and land the connection setup signalwithin the middle of the specified timing window. By such an approach,this can allow for any inaccuracy in the timing advance applied, toensure not only that the entire connection setup signal is receivedbefore the end of the timing window, but also that no portion of thatconnection setup signal is received before the start of the timingwindow.

It should be noted that the above coarse timing advance scheme can beapplied to a wide variety of different communication schemes, forinstance both TDD (time division duplex) and FDD (frequency divisionduplex) schemes. When employing an FDD scheme, the above-mentionedreceive/transmit timing constraint issue may not apply as the antennasystem can transmit and receive simultaneously, and hence steps 1210 and1220 of FIG. 14 will not be employed.

Using the above described techniques, it is possible to establish acommunication link with the ground terminal, even in situations wherethe separation distance between the aircraft 10 and the ground terminal20 exceeds that supported using the standard RACH mechanism. However, asillustrated schematically in FIG. 15, a further problem that can ariseis ensuring that in the subsequent uplink communications from theaircraft to the ground station 10 (using the cumulative timing advanceobtained by combining the initial timing advance chosen by the vehicleterminal 100 with the fine timing advance specified in the RACHresponse), the earlier-mentioned receive/transmit timing constraint isnot violated. In particular, as shown in FIG. 15, the communicationframe format provides multiple sub-frames that can in principle be usedfor uplink communication, namely the sub-frames U0, U1 and U2 shown inthe communication frame 1250. However, as indicated by the combinationof the lines 1255 and 1260, if the scheduling circuitry 140 within theground terminal 130 chooses to allocate resource blocks to the aircraft10 within either the U0 or the U1 sub-frames, then if the aircraftseparation distance from the ground terminal exceeds a schedulingthreshold distance (in this example the scheduling threshold distancebeing 100 km), then the receive/transmit timing constraint would beviolated.

In the example of FIG. 15, it is assumed that the separation distancebetween the aircraft 10 and the ground terminal 20 is 300 km, and hencefrom the earlier discussed FIG. 12A it will be understood that a timingadvance of approximately 2 ms may be specified. However, this wouldoverlap the sub-frames U0 and U1 with the downlink sub-frame DO and thespecial sub-frame S0, and as discussed earlier the special sub-frame S0may include some symbols transmitting downlink information. At any pointin time, the antenna system 105 can only be configured for downlinkcommunication or uplink communication, so this would violate thereceive/transmit timing constraint, even though, as indicated by theline 1265, that timing advance would correctly align the uplinkcommunications so that they are received in the relevant sub-frames U0,U1, U2 as per the timing employed by the ground terminal 130.

FIGS. 16A and 16B provide a flow diagram illustrating steps that can beperformed by the ground terminal when determining how to schedulesub-frames to the vehicle terminal, in order to resolve the issueillustrated in FIG. 15. At step 1300, the ground terminal will awaitreceipt of a connection setup signal, i.e. the earlier discussed RACHsignal, from the vehicle terminal. Then, at step 1305 the groundterminal determines whether to allow the vehicle terminal 100 toestablish a communication link with it. As discussed earlier, a numberof criteria can be assessed here. For example, the quality of thecommunication link can be assessed, and factors such as other vehicleterminals that are seeking to establish a communication link can beconsidered when deciding whether to accept the establishment of acommunication link with the vehicle terminal 100.

At step 1310, it is then concluded whether a communication link is to beestablished or not, and if not then at step 1315 the connection setuprequest is merely ignored. As will be apparent from the earlierdiscussed FIG. 11, this will result in no response being received by thevehicle terminal within a specified timeout period, and accordingly thevehicle terminal will proceed to step 1090 in order to seek to establisha communication link at a future time, either with that ground terminal130, or with another ground terminal.

Assuming it is decided at step 1310 that a communication link is to beestablished, then at step 1320 the communication link establishing andscheduling circuitry 140 computes a timing advance required based on thereceived connection setup signal. In particular, based on the placementof the received RACH signal within the RACH timing window, a timingadvance can be computed, this being the fine timing advance discussedearlier. At this stage, the computation performed by the communicationlink establishing and scheduling circuitry 140 does not need to takeaccount of the actual separation distance between the aircraft and theground terminal, since as discussed earlier that fine timing advancewill be combined with any coarse timing advance initially chosen by theaircraft when sending the RACH signal, in order to determine the fulltiming advance to be used for subsequent uplink communication.

However, as discussed earlier care needs to be taken when schedulinguplink sub-frames for the aircraft to ensure that the receive/transmittiming constraint is not violated, and to assist in this process theground terminal 130 does need to determine the separation between thevehicle terminal 100 and the ground terminal.

Accordingly, at step 1325 the ground terminal is arranged to determinethe location of the vehicle terminal. In particular, the distancecomputation circuitry 145 discussed earlier in FIG. 3 can accessinformation in order to determine the current position of the aircraft10. There are a number of ways in which the vehicle location informationcan be obtained, but in one example a flight tracking website may beaccessed in order to obtain current coordinate information. Thereafter,at step 1330 the separation distance between the ground terminal and thevehicle can be determined. In particular, the location of the groundterminal 130 will be fixed, and accordingly can be used when computingthe separation distance.

Then, at step 1335, one or more uplink sub-frames are allocated for useby the vehicle terminal taking into account the separation distance, soas to avoid violation of the receive/transmit timing constraint. Inparticular, in one example arrangement there may be multiple sub-framesthat can be allocated for uplink communication, such as the threesub-frames U0, U1, U2 discussed earlier. Which of those sub-frames isused when allocating uplink resource for the aircraft 10 can takeaccount of the separation distance. This will be discussed in moredetail later by way of example with reference to FIGS. 17A to 17C.However, from the earlier-discussed FIG. 15, it will be appreciated thatin the particular example chosen in FIG. 15 the scheduling circuitrycould avoid allocating resource blocks within the sub-frames U0 and U1,so that the aircraft is only allocated resource blocks within thesub-frame U2, such that when the timing advance is applied thereceive/transmit timing constraint will not be violated.

As indicated at step 1340, downlink sub-frames are also allocated to beused by the vehicle terminal for downlink communication from the groundstation to the aircraft.

Once the uplink and downlink sub-frames have been allocated, then theresponse signal can be issued to the vehicle terminal at step 1345identifying both the timing advance determined earlier at step 1320, andthe uplink and downlink sub-frames that are to be used for subsequentcommunication with the aircraft.

FIGS. 17A to 17C illustrate how uplink resource can be scheduled,assuming the communication frame format is as discussed earlier in FIG.2, and accordingly there are three sub-frames that can in principle beused for uplink communication. As indicated in FIG. 17A, where it isdetermined that the aircraft 10 is at 300 km from the relevant groundterminal 20, the propagation delay is 1 ms, and accordingly thecommunication frame 1350 as transmitted by the ground terminal isreceived as shown by the line 1355, such that the communication is onesub-frame out relative to the transmission timing. In this example, itis assumed that the scheduling circuitry determines at step 1335 toallocate the U2 sub-frame to the vehicle terminal for use in uplinkcommunication. As a result, as indicated by the line 1360, when thecumulative timing advance of 2 ms is applied, the downlink/uplink timingconstraint is not violated. Hence, the uplink communication can beperformed using this timing advance, and will ensure that it iscorrectly received by the ground terminal in the U2 sub-frame, asindicated by the line 1365. The approach shown in FIG. 17A can be usedwherever the separation distance exceeds 200 km, provided the separationdistance does not exceed 300 km.

FIG. 17B illustrates a scheduling approach that can be used when theseparation distance is between 100 and 200 km. Again, the communicationframe 1370 is transmitted from the ground terminal 20, and in thisspecific example it is assumed that the separation is 150 km, and hencethe delay in receiving the communication frame is 0.5 ms as shown by theline 1375. In this scenario, the cumulative timing advance that willapplied after the RACH sign-up process has been completed will be 1 ms.As a result, it is possible to accommodate uplink allocations in eitheror both of sub-frames U1 and U2 without violating the downlink/uplinktiming constraint, as indicated by the line 1380. As shown by the line1385, uplink communications in either of those two sub-frames will thenbe correctly received by the ground terminal 20.

FIG. 17C illustrates a scheduling scheme that can be used when theseparation distance is less than 100 km. The communication frame 1390 istransmitted from the ground terminal, and in this instance it is assumedthat the separation delay is 0.17 ms, this assuming the separationdistance is 50 km. In this instance, any of the three uplink sub-framesU0, U1 or U2 can be allocated for uplink communication, since thecumulative timing advance after the RACH process has been performed willbe 0.33 ms.

As shown by the line 1400, if the sub-frame U0 is used, this will causesome overlap of the U0 sub-frame transmission timing with the S0 frame.However, the extent of overlap still leaves some gap, and in particulardoes not overlap with any symbols within the S0 sub-frame that will beused for downlink communication, and accordingly the receive/transmittiming constraint is not violated. Further, as shown by the line 1405,any uplink communication of the three sub-frames U0, U1 or U2 will becorrectly received by the ground terminal with the appropriate timing.

It is anticipated that the traffic between an aircraft and a connectedground terminal will be heavily downlink centric, for example to supportthe earlier-mentioned Wi-Fi connectivity for passengers within theaircraft. As will be apparent from the earlier-discussed frame format ofFIG. 2, when using that frame format three sub-frames are reserved foruplink communication. This is required to allow for effective schedulingof uplink communications for aircrafts up to 300 km away from the groundterminal. However, in one example implementation the base station may beprovided with the flexibility to alter the communication frame formatunder certain conditions, in order to allow for a larger proportion ofthe communication frame to be used for downlink traffic when possible.

FIG. 18 illustrates three example communication frame formats that maybe used, each of which are supported LTE TDD (Time Division Duplex)frames. The frame format FC3 1410 is the format discussed earlier withreference to FIG. 2. The format FC4 1415 has one less uplink sub-frameand one more downlink sub-frame. Further, the frame format FC5 1420 hasonly a single uplink sub-frame, and an additional downlink sub-framerelative to the frame format FC4.

From the earlier scheduling examples illustrated with reference to FIGS.17A to 17C, it will be appreciated that it is only when the separationdistance exceeds 200 km (referred to in FIG. 18 as long range (LR)) thatthere is a need to schedule uplink communication in the last of thethree uplink sub-frames, and hence the requirement to use communicationframe FC3. When the distance is between 100 and 200 km (referred to inFIG. 18 as medium range (MR)), then uplink communication can bescheduled in the second uplink sub-frame, and hence it would still bepossible to schedule uplink communications even if the communicationframe format FC4 was used. Similarly, it will also be appreciated thatif the communication frame format FC4 is used, uplink communication withaircraft up to 100 km away (referred to in FIG. 18 as short range (SR))can also be accommodated when using the communication frame format FC4.

Finally, it will be appreciated that if the aircraft is less than 100 kmaway, then the communication frame format FC5 could be used, sinceuplink communication can be scheduled in the first uplink sub-frame(which happens to be the only uplink sub-frame in the frame format FC5).

FIG. 19 is a flow diagram illustrating how the ground terminal couldmake use of the three communication frame formats shown in FIG. 18 inorder to facilitate a higher downlink capacity when the location of theconnected aircrafts permits. At step 1450, it is determined whether allof the aircraft connected to that ground station are within the mediumor short ranges. If not, then the communication frame FC3 is used atstep 1455, and the process returns to step 1450.

However, if all of the connected aircraft are within the medium or shortrange, then the process can proceed to step 1460 where the aircraftterminal can switch to using communication frame FC4. A broadcast signalcan be sent from the ground terminal to all of the connected aircraftterminals to advise them of the change in the communication frame. Oncestep 1460 has been implemented, it will be appreciated that there is anadditional downlink sub-frame available when compared with thecommunication frame FC3.

Following step 1460, it can be determined at step 1465 whether allconnected aircraft are within the short range. If not, it is thendetermined at step 1470 whether there is a desire to connect with anaircraft exceeding the medium range. For example, the ground terminalmay receive a RACH signal from an aircraft within the long range seekingto establish a connection, and the ground terminal may decide that itwishes to service that request. Alternatively, it may be known that oneof the already connected aircraft is about to leave the medium rangeinto the long range, and it may be desirable to maintain connection withthat aircraft. If it is determined at step 1470 that there is desire toconnect with an aircraft exceeding the medium range, then the processproceeds to step 1455 where a switch is made to using the communicationframe FC3. Again, a broadcast signal can be sent from the ground stationto identify this change in the communication frame.

However, if at step 1470 it is determined that there is no desire toconnect with an aircraft exceeding the medium range, then the processcan merely return to step 1460.

If at step 1465 it is determined that all of the connected aircraft arewithin the short range, then the process can proceed to step 1475 wherethe communication frame FC5 can be used. Again, a broadcast signal canbe sent from the ground terminal to advise of the change in thecommunication frame format.

Following step 1475, it can be determined at step 1480 whether there isa desire to connect with an aircraft exceeding the short range. If not,the process merely returns to step 1475 where the communication frameformat FC5 continues to be used. However, if at step 1480 it isdetermined that there is a desire to connect with an aircraft exceedingthe short range, then the process proceeds to step 1470 where theearlier-discussed analysis is performed.

Accordingly, by such an approach, it can be seen that the groundterminal can make use of multiple communication frame formats so as toseek to maximum the downlink capacity available, taking into account theseparation between that ground terminal and the relevant aircraft. Thiscan further improve capacity within the network.

In one example implementation where lookup tables are used to determineinitial timing advances to be applied for RACH signals, those lookuptables can be updated as necessary dependent on the communication frameformat currently being employed by the ground terminal.

FIG. 20 is a block diagram illustrating the use of a high velocitymobility manager (HVMM) 1530 to perform a handover analysis process usedto control the handover decisions made within a radio access network1520. It is assumed in this example that the radio access network 1520is a wireless network conforming to the 4G (LTE) telecommunicationsStandard. An evolved packet core 1555 is used in the standard manner toimplement a framework for providing converged voice and data on a 4G(LTE) network, and in particular unifies voice and data on an internetprotocol (IP) service architecture with voice being treated as anotherIP application. In the example shown the various ground base stations1505, 1510, 1515 (also referred to as ground terminals) of the radioaccess network are coupled to the evolved packet core 1555 via theInternet 1525. Only three ground base stations are shown for ease ofillustration, but it will be appreciated that there will typically besignificantly more ground base stations within the network.

Within a moving vehicle that is to make use of the radio access network1520, a vehicle terminal 1500 is provided which may comprise one or moreitems of user equipment 1502 with an associated antenna to performwireless communication with corresponding base station nodes 1507, 1512,1517 (also referred to herein as eNodeBs or eNBs) within the groundterminals 1505, 1510, 1515. Control circuitry (also referred to hereinas ATG (Air to Ground) agents 1504, 1509, 1514, 1519) can also beprovided within the vehicle terminal 1500 and ground terminals 1505,1510, 1515 to control the operation of the connected radio accessnetwork components. Hence, by way of example, IP packets may beexchanged between the high velocity mobility manager (HVMM) 1530 and theATG agents 1509, 1514, 1519 within the ground terminals 1505, 1510, 1515via the Internet 1525, in order to control and/or influence handoverprocedures performed within the radio access network, and suchinformation can also be propagated on via the radio access network tothe ATG agent 1504 within the vehicle terminal 1500. In addition thevehicle terminal ATG agent 1504 can report handover metrics to the HVMM1530 if they are available, with these metrics for example beingobtained from information systems on board the vehicle.

The high velocity mobility manager 1530 provides a centralised resourcefor computing handover metrics that may be used to determine anappropriate target base station when performing a handover procedure totransition communication with the moving vehicle from a currentconnected base station (for example, the base station 1505 in theexample of FIG. 20) to that target base station (which may for examplebe one of the other base stations 1510, 1515 shown in FIG. 20). Whilstonly a single vehicle terminal 1500 is shown in FIG. 20 for simplicity,it will appreciated that in a normal ATG network, there may be multipleaircraft connected to the wireless network, and the HVMM 1530 cangenerate handover metrics for each of those connected aircraft.

It should also be noted that whilst an aircraft is given as an exampleof a moving vehicle to which the techniques described herein may beapplied, the techniques can be applied to other types of movingvehicles, for example a train, where the ground terminals may typicallybe spread out along the track.

As shown in FIG. 20, the HVMM 1530 includes base station locationidentification circuitry 1535 used to obtain base station locationinformation for a variety of base stations that provide the wirelessnetwork 1520. Typically the base stations will be fixed, and accordinglytheir location information will be fixed. Hence, in one exampleimplementation the base station location identification circuitry maytake the form of a database storing the location coordinates of each ofthe base stations forming the radio access network.

In addition, the HVMM 1530 includes moving vehicle tracking circuitry1540 for obtaining position and velocity information for the movingvehicle. In some implementations, the moving vehicle tracking circuitry1540 may also obtain bearing information indicative of the direction inwhich the aircraft is pointing. It will be appreciated that due toprevailing winds the direction of travel of the aircraft may differ tothe bearing, and in some instances it can be useful to have knowledge ofthe bearing of the aircraft, as for example will be discussed later withreference to the calculation of antenna gain metrics.

There are a number of ways in which the moving vehicle trackingcircuitry 1540 may obtain the relevant information about the movingvehicle. For example, that information may be reported directly via thevehicle terminal through the radio access network 1520 to the connectedbase station 1505, from where it can be reported back to the movingvehicle tracking circuitry. However, alternatively the moving vehicletracking circuitry may be able to obtain this information from flighttracking facilities such as a flight tracking website. In particular,flight tracking systems already exist that can be accessed to provideinformation such as the position and velocity of the aircraft.

As shown in FIG. 20, the handover metrics computation circuitry hasaccess to both the base station location identification circuitry 1535and the moving vehicle tracking circuitry 1540, and is arranged togenerate at least one handover metric computed from the position andvelocity information for the moving vehicle and the base stationlocation information. Each generated handover metric can then bereferenced when determining a target base station to be used whenperforming a handover procedure to transition communication with themoving vehicle from a current base station to the target base station.

The handover metrics generated by the handover metrics computationcircuitry can be used in a variety of ways. For example, those handovermetrics could be reported in suitable IP packets transferred via theInternet to the currently connected ground terminal for use inevaluating a suitable target base station to which communication shouldbe transferred during a handover procedure. However, in one exampleimplementation the HVMM 1530 also includes handover decision circuitry1550 for making handover decisions based on the handover metricsgenerated by the handover metrics computation circuitry 1545. In such animplementation, the handover decision circuitry can then be arranged toissue a signal via the Internet 1525 to the connected ground terminal1505 to initiate a forced handover from the current ground terminal to atarget ground terminal identified by the handover request issued by thehandover decision circuitry 1550. Existing mechanisms can then be usedto perform the handover procedure, for example a “blind handover”mechanism.

FIG. 21 is an aerial view schematically illustrating an aircraft 1560travelling across a network of ground base stations arranged in agenerally grid-like manner. Given that the handover metrics computationcircuitry has access to base station location information, and to thecurrent position and velocity information for the aircraft 1560, it candetermine separation vectors between the aircraft 1560 and each of aplurality of ground base stations within the wireless network, theseparation vectors being denoted in FIG. 21 by the references d1 to d7.

In the illustrated example the aircraft 1560 is travelling in thedirection 1562 with velocity v. It will be appreciated that, once theseparation vectors have been determined, and given that the velocity isknown, the handover metrics computation circuitry can determine thecomponent of the velocity along each separation vector. As will bediscussed in more detail later, this information can be used in thedetermination of one or more of the handover metrics computed by thehandover metrics computation circuitry.

As shown in FIG. 21, the bearing of the aircraft 1560 is denoted by thedotted line 1564 and an angular separation between the bearing 1564 andthe direction of travel 1562 is shown as the angle α in FIG. 21. Thebearing information can be useful when determining one or more of thehandover metrics, as will be discussed in more detail later for examplewith reference to FIG. 25.

A number of criteria can be assessed by the handover decision circuitry1550 when making handover decisions, and corresponding handover metricsfor use when applying those criteria can be produced by the handovermetrics computation circuitry 1545. One example criteria that may beused is illustrated schematically in FIG. 22. In particular, in thisexample, it may desired to minimise the Doppler effect by seeking toclassify the various candidate target base stations by the relativespeed between those base stations and the vehicle. As shown in FIG. 22,the aircraft 1560 is moving in the direction 1565. FIG. 22 schematicallyillustrates an aerial view looking down on the aircraft and theunderlying base stations, and in particular shows three base stations1570, 1575, 1580. One of these base stations may be the base stationthat the aircraft is already connected to, whilst the others arecandidate target base stations to which communication might betransferred during a handover operation. Alternatively all of the basestations shown may be candidate target base stations.

As shown schematically in FIG. 22, separation vectors 1572, 1577, 1582can be computed for each of the base stations, providing an indicationof the direction of separation between the aircraft and the various basestations. A component of the velocity can then be computed along withvarious separation vectors, in order to identify the relative speedbetween the aircraft 1560 and each of the base stations 1570, 1575,1580. As the relative speed between the aircraft and a base stationincreases, then so the Doppler effect will increase. Accordingly, purelyby way of example, it will be appreciated that the base station 1575,although further away than the base station 1570, has a lower relativespeed between it and the aircraft than does the base station 1570. Bytaking relative speed into account when assessing an appropriatecandidate target base station to transfer communication to during ahandover process, this may increase the likelihood of selecting basestation 2 1575 rather than base station 1 1570 in this instance.

Whilst the relative speed can be a signed value, hence identifyingwhether the aircraft is moving towards the base station or away from thebase station, in one example implementation it is only the magnitude ofthe relative speed that is of interest to the first criteria shown inFIG. 22, and hence the sign information may be ignored.

A second criterion that may be considered when making the handoverdecision is illustrated schematically in FIG. 23. In particular, it maybe desirable to seek to minimise the effect of fast changes of Dopplerfrom positive to negative by classifying the base stations in terms ofthe speed of change of the Doppler effect. FIG. 23 schematicallyillustrates two base stations 1590, 1595 that the aircraft 1560 ismoving towards. In order to enable criterion 2 to be assessed, thehandover metrics computation circuitry 1545 may generate a Dopplereffect metric indicating a speed of change of the Doppler effect ontransmitted signals between the moving vehicle and a candidate targetbase station. This metric can be produced by computing a change inrelative speed between the moving vehicle and the candidate target basestation. From FIG. 23, it will be appreciated that the change inrelative speed is more marked in relation to base station 1590 than itis in relation to base station 1595. Hence, whilst base station 1590 isnearer to the aircraft 1560, the Doppler effect may be changing morerapidly, whereas for the base station 1595 the Doppler effect may bechanging less rapidly. Hence, even though the base station 1595 isfurther away, assessment of criterion 2 may increase the likelihood ofbase station 1595 being selected as the target base station for thehandover process.

FIGS. 22 and 23 illustrate two criteria that seek to compensate for theDoppler effect caused by the relatively high velocity of the aircraft.However, another issue that can arise within air to ground systemsresults from the relatively large distances that may exist between thebase stations, and hence the relatively large separation distance thatmay exist between the aircraft and any particular candidate target basestation. As will be apparent from the earlier discussions, the aircraftterminal may need to perform timing advance operations with regards tothe signals that it transmits in order to ensure that those signals arereceived at the connected ground terminal at an appropriate timing. Withregards to the uplink communications from the aircraft to the connectedground base station, it will be appreciated for example from the earlierdiscussion of FIG. 18 that the number of sub-frames available for uplinkcommunication may be reduced as the separation distance increases, andhence this can lead to a loss of uplink capacity as the separationdistance increases.

Accordingly, another criterion that can be used when making handoverdecisions is the criterion 3 schematically illustrated in FIG. 24, wherethe aim is to seek to minimise the loss of uplink capacity as a resultof the uplink timing advance algorithms that may need to be employedwithin the system. As shown in FIG. 24, when the separation distancebetween the aircraft 1560 and a ground base station 1600 is relativelylarge, referred to herein as long range (LR), then there may be only asingle uplink sub-frame 1602 available. However, when the separationdistance reduces to medium range (MR), there may be two uplinksub-frames 1602, 1604 available. Further, if the separation distancereduces to short range (SR) then there may three uplink sub-frames 1602,1604, 1606 available.

The handover metrics computation circuitry 1545 may be arranged togenerate an uplink capacity metric indicative of the available uplinkcapacity, and in particular taking into account knowledge of the numberof uplink sub-frames available for uplink communication. As will bediscussed in more detail later, other information can also be factoredin to the generation of such uplink capacity metrics. For example, thehandover metrics computation circuitry may receive capacity reports fromthe candidate target base stations indicative of existing uplinkcapacity utilisation, and then factor that information into theproduction of the uplink capacity metric. For instance, if one candidatetarget base station already has a significant number of aircraftsconnected to it, then even though it may be relatively close to theaircraft for which the handover metrics are being produced, and hencethere may be for example three uplink sub-frames available, it may bethat those three uplink sub-frames are relatively highly utilised.Conversely, a candidate target base station that is at a largeseparation distance may only have one uplink sub-frame available, butmay be lightly utilised, due for example to a low number of aircraftbeing connected to that base station. Hence, whilst the uplink capacitymetric could be computed based purely on the separation distance, andhence an understanding of the number of uplink sub-frames available, itcan in some implementations also take into account an indication of theactual capacity utilisation when determining a suitable uplink capacitymetric for a particular candidate target base station. Hence, thisallows the indication of the number of available uplink sub-framesdetermined based on the separation distance to be normalised taking intoaccount the available capacity.

Another criterion that can be considered when seeking to make handoverdecisions is a prediction of expected received signal power for eachcandidate target base station. It has been found that the handovermetrics computation circuitry can be arranged to generate a handovermetric that provides a prediction of expected signal strength, butwithout needing to receive any signal strength metric reports from theaircraft. In particular, based on bearing information for the aircraft,and knowledge of the antenna deployment within the aircraft, it has beenfound that the handover metrics computation circuitry can generate asuitable metric indicative of expected signal strength. This isillustrated schematically in FIG. 25.

In FIG. 25, the aircraft is travelling at a velocity V1 along thedirection 1607. However, two different bearings of the aircraft areconsidered. In a first example, the aircraft is orientated as shown bythe reference numeral 1560, and has hence adopted a bearing αB1 relativeto north. In a second example, the aircraft is orientated as shown bythe reference numeral 1560′ and has hence adopted the bearing αB2relative to north. The actual direction of travel relative to north isgiven by the angle αV1, and this is the direction of travel for bothexample orientations shown in FIG. 25 (it being assumed that prevailingwind conditions are different between the two orientations shown, henceresulting in the different bearings adopted for those two exampleorientations).

Also shown in FIG. 25 are six example ground base stations. In thisexample implementation, it is assumed that the handover metricscomputation circuitry is provided with antenna configuration informationfor the aircraft, and hence has sufficient knowledge to know the form ofthe beam patterns that will be produced by the antennas on the aircraft.It will be appreciated that the antennas will typically be at a fixedlocation on the aircraft, but may be provided with various beam steeringcapabilities, whether those be mechanical and/or electronic beamsteering capabilities. A number of different modes may be provided forthe beam patterns, but with knowledge of the mode being deployed by theaircraft, the handover metrics computation circuitry will have knowledgeof the beam patterns produced by the antennas on the aircraft.

In the example shown in FIG. 25, a simple shape of beam typical for asingle antenna is shown, and in particular for the aircraft position1560 two beam lobes 1605, 1610 are shown. In particular, it will beappreciated that in this example beams are formed on each side of theaircraft. If instead the aircraft adopts the orientation 1560′ then thecorresponding beams become 1605′ and 1610′.

Accordingly, it can be seen that if the separation vector for each ofthe candidate target base stations is computed in the manner discussedearlier, and the bearing and antenna configuration for the aircraft areprovided, the handover metrics computation circuitry can predict anantenna gain for each of the ground base stations. Indeed, with thisinformation it can be determined whether there are any ground basestations that are likely to be out-of-range or have a very poor antennagain. In particular, in the example of FIG. 25 it will be appreciatedthat when the aircraft adopts the orientation 1560, the ground basesstations GBS2, GBS3 and GBS5 will be within the coverage area and aprediction of the antenna gain can be made for each of those ground basestations. However, the same analysis will identify that ground basestations GBS1, GBS4 and GBS6 will not be suitable candidate target basestations. Conversely, if the orientation 1560′ is adopted, the groundbase stations GBS1, GBS4 and GBS6 become suitable candidate target basestations, but the candidate target base stations GBS2, GBS3 and GBS5 arenot suitable.

Hence, the prediction of the antenna gain using this approach can beparticularly useful. Not only can it provide antenna gain informationfor a number of candidate target base stations, it can also effectivelybe used to discount certain candidate target base stations for whichthere is unlikely to be an accepted minimum level of signal strength.

FIG. 26 is a flow diagram illustrating the handover analysis processperformed by the handover metrics computation circuitry 1545 of FIG. 20in one example implementation. At step 1650, an analysis trigger isawaited. The analysis trigger can take a variety of forms, but in oneexample implementation the performance of the analysis may be triggeredperiodically. In the ATG system envisaged, handover operations inrespect of any particular aircraft are likely to happen relativelyinfrequently due to the large distances between the ground basestations, and hence the performance of the analysis periodically foreach aircraft will typically be sufficient to produce suitable handoverinformation to be used at the time a handover is required.

At step 1655, tracking information for the aircraft is obtained, via themoving vehicle tracking circuitry 1540 shown in FIG. 20. Thisinformation will provide the current position and the current velocityof the aircraft, and optionally may also provide the bearing informationdiscussed earlier.

At step 1660, a determination is made of the base stations to considerfor the analysis process. This will typically include the current basestation to which the aircraft is connected, plus a number of candidatetarget base stations. The candidate target base stations can bedetermined from network neighbourhood information maintained within thewireless network. This may for example include a plurality of candidatetarget base stations located within a geographical region around thecurrent connected base station. That default network neighbourhoodinformation could potentially be altered at step 1660 to discount one ormore candidate target base stations, for example by taking into accountthe direction of travel and/or bearing of the aircraft.

The process then proceeds to step 1665 where a parameter i is set equalto 0, and then the process proceeds to step 1670 where a handover metrici is computed for each considered base station. As will be apparent fromthe earlier discussions, the handover metrics computation circuitry canactually generate a variety of different handover metrics, for examplefour different types of handover metrics to cover the four differentcriterion discussed earlier with reference to FIGS. 22 to 25. Hence,during each pass through step 1670, the handover metrics computationcircuitry will compute a particular one of those handover metrics foreach of the considered base stations. This process will be discussed inmore detail later for the four example metrics discussed earlier, withreference to the flow diagrams of FIGS. 27 to 30.

At step 1675, it is determined whether the parameter i is equal to amaximum value, i.e. whether all of the different handover metrics haveyet been generated, and if not the parameter i is incremented at step1680 and the process then returns to step 1670.

Once all of the handover metrics have been computed, the processproceeds from step 1675 to the optional step 1685. Here, a filteringoperation may be performed based on the computed handover metrics. Forexample, configurable valid ranges of values for each of the computedmetrics may be specified, and the filtering operation can determinewhether the computed handover metrics are within those valid ranges. Iffor a particular candidate target base station at least one of thecomputed handover metrics is outside of the acceptable range of values,it can be decided at this step to discard that candidate target basestation from further consideration. Hence, purely by way of example withreference to criterion 4 discussed earlier with reference to FIG. 25, itmay be determined that the antenna gain metric computed for a particularcandidate target base station is so poor that that candidate target basestation should be excluded from further consideration.

Following step 1685, or directly following step 1675 if the filteringoperation is not performed, the process proceeds to step 1690 where thevarious computed handover metrics for each considered base station (oreach remaining considered base station after the filtering operation ifthe filtering operation is performed) are combined in order to produce ahandover result for that considered base station. It will be appreciatedthat the various handover metrics will have different ranges of values,and accordingly prior to combining the metrics those metrics can benormalised so as to effectively put them onto the same scale withrespect to each other. For example, each of the computed metrics couldbe converted to a percentage value between 0 and 100, or a fractionvalue between 0 and 1. In addition, it is possible to weight differenthandover metrics differently with respect to each other, so as toincrease the importance of certain metrics over other metrics. Hence, asshown by step 1690, the handover metrics can be normalised and weightedprior to those handover metrics then being combined to produce thehandover result for each considered base station.

FIG. 27 is a flow diagram illustrating a process performed by thehandover metrics computation circuitry 1545 to produce a Doppler effectmetric useful in assessing criterion 1 discussed earlier. At step 1700,a parameter y is set equal to 0. Then, at step 1705 the location of basestation y is obtained, whereafter at step 1710 a separation vector forbase station y is computed based on the current location of theaircraft.

Thereafter, at step 1715, a component of the velocity of the aircraftalong the separation vector is calculated, this also being referred toherein as the relative speed between the aircraft and the base station.That relative speed is then output as a handover metric for base stationy at step 1720, whereafter at step 1725 it is determined whether thereare any more base stations to consider. If so, y is incremented at step1730, and the process returns to step 1705. Once all of the basestations have been considered, the process ends at step 1735.

It will be appreciated that whilst the process of FIG. 27 is shown as asequential process, in alternative implementations it may be possible tocompute the handover metric for each of the base stations in parallel.

FIG. 28 is a flow diagram illustrating steps that may be performed bythe handover metrics computation circuitry in order to generate aDoppler effect metric useful in assessing criterion 2 discussed earlier.At step 1750, a parameter y is set equal to 0, and then at step 1755 thelocation of base station y is obtained. At step 1760, the separationvector between the aircraft and the base station is computed using thelocation determined at step 1755 and the current location informationfor the aircraft, and then the relative speed is calculated, for exampleby implementing the process discussed earlier with reference to step1715 of FIG. 27. This relative speed will be referred to herein as RS₁.

At step 1765, the relative speed is recomputed after a time interval ΔT.This second relative speed will be referred to herein as RS₂.

At step 1770, a change in the relative speed is computed by subtractingthe second relative speed RS₂ from the first relative speed RS₁, anddividing that result by the time difference between the two relativespeed computations. The resultant computed change in relative speed isthen output as a handover metric for base station y at step 1775,whereafter at step 1780 it is determined whether there are any more basestations to consider. If so, the value of y is incremented at step 1785and the process returns to step 1755. Once all base stations have beenconsidered, the process ends at step 1790.

Again, whilst the process shown in FIG. 28 is illustrated as asequential process performed for each base station, it will beappreciated that in some implementations it may be possible to computethe handover metric for each base station in parallel.

FIG. 29 is flow diagram illustrating steps performed by the handovermetrics computation circuitry to compute an uplink capacity metricuseful in assessing criterion 3 discussed earlier. At step 1800, aparameter y is set equal to 0, and then at step 1805 the location ofbase station y is obtained. At step 1810, the separation distance forbase station y is computed based on the current location of theaircraft. It should be noted that in this example a separation vector isnot needed, and it is merely the distance itself, rather than anydirection information, that is required.

At step 1815, it is determined whether the separation distance is abovea first threshold. A first threshold can be used to distinguish betweenlong range and medium range or less, the terms long range, medium rangeand short range having been discussed earlier, see for example thediscussion of FIGS. 17 and 18.

If the separation distance is above the first threshold, then it isdetermined at step 1820 that the aircraft is at a long range from thebase station y, and that hence the number of uplink sub-frames availableto use will be one. This is due to the timing advance mechanism employedto allow communication at such long ranges, as discussed earlier withreference to FIGS. 17 and 18.

If the separation distance is not above the first threshold, then theprocess proceeds to step 1825 where it is determined whether theseparation distance is above a second threshold, this being a thresholdbetween the medium range and short range. If the separation distance isabove the second threshold, then at step 1830 it is decided that thelocation of the aircraft is in the mid range relative to the basestation, and that hence the number of uplink sub-frames will be 2.However, if the separation distance is below the second threshold, thenit is determined at step 1835 that the aircraft is at a short rangedistance from the base station, and that hence the number of sub-frameswill be 3.

Following steps 1820, 1830 or 1835, then optionally at step 1840 theuplink capacity metric (in this case indicating whether the number ofuplink sub-frames is 1, 2 or 3) can be adjusted based on capacity reportinformation from base station y. In particular, the base station y mayprovide a capacity report, for example identifying available uplinkcapacity. This allows for a more informed uplink capacity metric to begenerated, by normalising the uplink capacity report indicative ofavailable uplink capacity taking into account the number of availableuplink sub-frames. Purely by way of example, if one base station isdetermined to be at short range, and hence provides 3 uplink sub-framesfor the aircraft, whilst another base station is at long range, andhence only provides 1 uplink sub-frame, that long range base station maystill ultimately provide just as good or better uplink capacity if it isrelatively lightly loaded relative to the short range base station.

Following step 1840, or directly following steps 1820, 1830, 1835 ifstep 1840 is omitted, it is determined at step 1845 whether there areany more base stations to consider, and if so the parameter y isincremented at step 1850 and the process returns to step 1805. Once allbase stations are considered, the process ends at step 1855.

As with the earlier discussed FIGS. 27 and 28, whilst the process ofFIG. 29 has been shown as a process performed sequentially for each basestation, it will be appreciated that the computations may be performedin parallel for each base station in certain implementations.

FIG. 30 illustrates a process that may be performed by the handovermetrics computation circuitry to generate an antenna gain handovermetric useful in assessing criterion 4 discussed earlier. At step 1860,a parameter y is set equal to 0, and then at step 1865 the location ofbase station y is obtained. At step 1870, the separation vector iscomputed based on a current location of the aircraft and the location ofthe base station obtained at step 1865. In addition, at step 1875 thebearing of the aircraft is obtained, along with the aircraft antennaconfiguration information. As discussed earlier, the aircraft antennaconfiguration information is sufficient to identify the pattern of beamproduced by the aircraft, and the orientation of that beam patternrelative to the longitudinal axis of the aircraft. Hence, armed withthat information and bearing information, it can be determined how thebeam pattern will project over the underlying surface of the earth.

At step 1880, an antenna gain metric is computed for base station ygiven the aircraft bearing, the aircraft antenna information (which mayinclude antenna beam configuration information) and the separationvector. The antenna beam configuration matters for electronically andmechanically steerable antennas where antenna gain pattern is modifiedby beam steering configuration. The antenna gain metric may becalculated based on the best possible gain in a given direction obtainedby applying the beam optimised for that direction i.e. offering maximumgain in that direction.

At step 1885, the computed antenna gain metric is output as a handovermetric for base station y, and then at step 1890 it is determinedwhether any more base stations need to be considered. If so, theparameter y is incremented at step 1895 and the process returns to step1865. Once all base stations have been considered, the process ends atstep 1897. Again, whilst the process of FIG. 30 is shown as beingperformed sequentially for each base station, in an alternativeimplementation the computation of the antenna gain metric may beperformed in parallel for each base station.

FIG. 31 illustrates how the handover result mentioned at step 1690 ofFIG. 26 can be computed for a particular base station BS_(x). In thisexample, the term Rx_(x) represents the normalised handover metric n forthe base station BS_(x). Hence, by way of example, if handover metricsare produced for each of the 4 criteria discussed earlier, then therewill be 4 normalised handover metrics Rx₀, Rx₁, Rx₂ and Rx₃ for basestation x. The terms α₁, α₂, α₃ and α₄ then indicate the weights appliedto each of those four handover metric. As discussed earlier, differentweights can be given to each of the handover metrics in order toprioritise some metrics over others if desired.

FIG. 32 is a flow diagram illustrating a process that may be performedby the handover decision circuitry 1550 of FIG. 20 in one exampleimplementation. Whilst in principle the generated handover metrics couldbe provided via suitable IP packet communication to the currentconnected ground terminal for use when making handover decisions, in theexample illustrated in FIG. 32 a forced handover process is insteadperformed, where the handover decision circuitry 1550 makes the handoverdecision, and then issues a request to the currently connected groundterminal to initiate a handover to a different ground terminal specifiedin that request.

At step 1900, it is determined whether there are any candidate targetbase stations having a better handover result than the currentlyconnected base station. If not, the process proceeds to step 1915, andno handover is initiated. Instead, the aircraft remains connected to thecurrently connected ground terminal.

However, assuming there is a least one candidate target base stationhaving a better handover result than the current base station, then theprocess proceeds to step 1905 where it is determined whether thehandover result for the best candidate target base station exceeds thehandover result for the current connected base station by a determinedthreshold. The determined threshold may be configurable, and can bechosen so as to avoid “thrashing” where handover occurs multiple timesbetween closely matched base stations. If this test is not passed, thenagain the process proceeds to step 1915 and handover is not initiated.However, if it is passed, the process proceeds to step 1910 where ahandover is initiated. At this point, the HVMM 1530 will issue a requestvia the Internet connection 1525 to the currently connected groundterminal (in the example of FIG. 20 this being the ground terminal 1505)informing the ground terminal to perform a handover procedure, with thatrequest specifying the ground terminal to which handover should beperformed.

Once the currently connected ground station has received the request toperform a forced handover, and the target base station to which thehandover should be performed has been identified by that request, then astandard handover procedure (called “blind handover”) can be employed inorder to perform the handover of the aircraft to the identified groundterminal.

Alternatively the metrics discussed here can be used to dynamicallyreconfigure the network neighbourhood on the serving base station. Inthis case the metrics are used to identify the best 2 or more targetbase stations and configure them as the neighbours. With the neighboursconfigured the system could use standard measurements and measurementbased handover.

The techniques described herein allow a number of different handovermetrics to be generated that can be used to enhance the algorithmsevaluating the decision to trigger the handover from one ground basestation to another ground base station for an air station terminal (AST)located within an aircraft. The generated metrics assist in makingsuitable handover decisions taking into account the high speed of theaircraft and the relatively long distances between the base stations.Further, the handover analysis circuitry and handover decision circuitrycan be centralised within the system, and can perform the generation ofthe handover metrics and the determination as to which ground basestation to handover communication to without needing to receive anyreports from the AST. In particular, all of the described metrics can begenerated based on information about the location of the base stations,and vehicle tracking information identifying the current location,velocity and optionally bearing of the aircraft. It has been found thatthe adoption of this technique provides a particularly efficient andeffective mechanism for controlling handover within ATG systems.

In the present application, the words “configured to . . . ” are used tomean that an element of an apparatus has a configuration able to carryout the defined operation. In this context, a “configuration” means anarrangement or manner of interconnection of hardware or software. Forexample, the apparatus may have dedicated hardware which provides thedefined operation, or a processor or other processing device may beprogrammed to perform the function. “Configured to” does not imply thatthe apparatus element needs to be changed in any way in order to providethe defined operation.

Although particular embodiments have been described herein, it will beappreciated that the invention is not limited thereto and that manymodifications and additions thereto may be made within the scope of theinvention. For example, various combinations of the features of thefollowing dependent claims could be made with the features of theindependent claims without departing from the scope of the presentinvention.

1. (canceled)
 2. An apparatus comprising: computation circuitry tocompute, for each moving vehicle within a group of moving vehiclesassociated with a given base station, a separation distance between thatmoving vehicle and the given base station; and communication frameformat selection circuitry to determine a selected communication frameformat to be used for communication between the given base station andeach moving vehicle within the group of moving vehicles, wherein theselected communication frame format is selected from a set ofcommunication frame formats, where the set of communication frameformats comprise communication frame formats defining associatedcommunication frames that differ from each other regarding a proportionof the communication frame available for downlink communication from thegiven base station to the moving vehicles within the group of movingvehicles, wherein the communication frame format selection circuitry isarranged to determine the selected communication frame format independence on the separation distances computed by the computationcircuitry for the moving vehicles within the group of moving vehicles.3. An apparatus as claimed in claim 2, wherein: the communication frameformat selection circuitry is arranged to determine, as the selectedcommunication frame format, the communication frame format from the setof communication frame formats that allows a greater proportion of thecommunication frame to be used for downlink traffic whilst alsosupporting, taking into account the separation distances computed by thecomputation circuitry for the moving vehicles within the group of movingvehicles, uplink communication to the given base station from the movingvehicles in the group of moving vehicles.
 4. An apparatus as claimed inclaim 3, wherein: a proportion of the selected communication frameformat that is required to support the uplink communication is dependenton a separation distance associated with the moving vehicle in the groupof moving vehicles that is furthest from the given base station.
 5. Anapparatus as claimed in claim 4, wherein the communication frame formatselection circuitry is arranged to determine the selected communicationframe format in dependence on which of a plurality of predeterminedranges the separation distance associated with the moving vehicle in thegroup of moving vehicles that is furthest from the given base stationfalls within.
 6. An apparatus as claimed in claim 4, wherein thecommunication frame format selection circuitry is arranged tore-determine the selected communication frame format at least when thereis a change in which predetermined range the separation distanceassociated with the moving vehicle in the group of moving vehicles thatis furthest from the given base station falls within.
 7. An apparatus asclaimed in claim 2, wherein the group of moving vehicles comprisesmoving vehicles that are currently connected to the given base station.8. An apparatus as claimed in claim 7, wherein the group of movingvehicles further comprises one or more moving vehicles that it isdesired to connect to the given base station.
 9. An apparatus as claimedin claim 2, wherein each communication frame comprises a plurality ofsub-frames, and each communication frame format in the set ofcommunication frame formats identifies a different number of sub-framesto be allocated for downlink communication than is identified by eachother communication frame format in the set of communication frameformats.
 10. An apparatus as claimed in claim 2, wherein thecommunication frame format selection circuitry is arranged to cause abroadcast signal to be sent to the moving vehicles in the group ofmoving vehicles to identify the selected communication frame format. 11.An apparatus as claimed in claim 2, further comprising: moving vehicletracking circuitry to obtain location information for each of the movingvehicles in the group of moving vehicles, wherein the computationcircuitry is arranged to compute, for each moving vehicle within thegroup of moving vehicles associated with the given base station, theseparation distance between that moving vehicle and the given basestation based on the location information for that moving vehicle asprovided by the moving vehicle tracking circuitry and base stationlocation information identifying a location of the given base station.12. An apparatus as claimed in claim 2, wherein: each moving vehicle inthe group of moving vehicles provides location information; and thecomputation circuitry is arranged to compute, for each moving vehiclewithin the group of moving vehicles associated with the given basestation, the separation distance between that moving vehicle and thegiven base station based on the location information for that movingvehicle as provided by the moving vehicle and base station locationinformation identifying a location of the given base station.
 13. Amethod of selecting a communication frame format for communication witha given base station, comprising: computing, for each moving vehiclewithin a group of moving vehicles associated with the given basestation, a separation distance between that moving vehicle and the givenbase station; employing communication frame format selection circuitryto determine a selected communication frame format to be used forcommunication between the given base station and each moving vehiclewithin the group of moving vehicles, wherein the selected communicationframe format is selected from a set of communication frame formats,where the set of communication frame formats comprise communicationframe formats defining associated communication frames that differ fromeach other regarding a proportion of the communication frame availablefor downlink communication from the given base station to the movingvehicles within the group of moving vehicles; and determining theselected communication frame format in dependence on the separationdistances computed for the moving vehicles within the group of movingvehicles.
 14. An apparatus comprising: computation means for computing,for each moving vehicle within a group of moving vehicles associatedwith a given base station, a separation distance between that movingvehicle and the given base station; and communication frame formatselection means for determining a selected communication frame format tobe used for communication between the given base station and each movingvehicle within the group of moving vehicles, wherein the selectedcommunication frame format is selected from a set of communication frameformats, where the set of communication frame formats comprisecommunication frame formats defining associated communication framesthat differ from each other regarding a proportion of the communicationframe available for downlink communication from the given base stationto the moving vehicles within the group of moving vehicles, wherein thecommunication frame format selection means is arranged to determine theselected communication frame format in dependence on the separationdistances computed by the computation means for the moving vehicleswithin the group of moving vehicles.